Artifact detection electrode

ABSTRACT

An intraoperative neurophysiological monitoring system includes an adaptive threshold detection circuit adapted for use in monitoring with a plurality of electrodes placed in muscles which are enervated by a selected nerve and muscles not enervated by the nerve. Nerve monitoring controller algorithms permit the rapid and reliable discrimination between non-repetitive electromyographic (EMG) events and repetitive EMG events, thus allowing the surgeon to evaluate whether nerve fatigue is rendering the monitoring results less reliable and whether anesthesia is wearing off. An artifact detection electrode provides a reliably detectable impedance imbalance between the electrode leads and antenna-like qualities for enhanced detection of current and electromagnetic artifacts.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of prior application Ser. No.09/985,708 filed Nov. 6, 2001 now U.S. Pat. No. 7,214,197, the entiredisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to surgical apparatus and moreparticularly to a neurological monitoring system including a nerveintegrity monitoring instrument for use in conjunction with one or moreelectrical stimulus probes as an intraoperative aid in defining thecourse of neural structures. The invention is particularly applicablefor use in monitoring facial electromyographic (EMG) activity duringsurgeries in which a facial motor nerve is at risk due to unintentionalmanipulation, although it will be appreciated that the invention hasbroader applications and can be used in other neural monitoringprocedures.

2. Discussion of the Prior Art

Despite advances in diagnosis, microsurgical techniques, andneurological techniques enabling more positive anatomical identificationof facial nerves, loss of facial nerve function following head and necksurgery such as acoustic neuroma resection is a significant risk. Nervesare very delicate and even the best and most experienced surgeons, usingthe most sophisticated equipment known, encounter a considerable hazardthat a nerve will be bruised, stretched or severed during an operation.Studies have shown that preservation of the facial nerve during acousticneuroma resection may be enhanced by the use of intraoperativeelectrical stimulation to assist in locating nerves. Very broadlystated, the locating procedure, also known as nerve integritymonitoring, involves inserting sensing or recording electrodes directlywithin cranial muscles enervated or controlled by the nerve of interest.A suitable monitoring electrode is disclosed in U.S. Pat. No. 5,161,533(to Richard L. Prass et al.), the entire disclosure of which isincorporated herein by reference

One method of nerve localization involves the application of electricalstimulation near the area where the subject nerve is believed to belocated. If the stimulation probe contacts or is reasonably near thenerve, the stimulation signal applied to the nerve is transmittedthrough the nerve to excite the related muscle. Excitement of the musclecauses an electrical impulse to be generated within the muscle; theimpulse is transferred to the recording electrodes, thereby providing anindication to the surgeon as to the location of the nerve. Stimulationis accomplished using hand held monopolar or bipolar probes such as theElectrical Stimulus Probe disclosed in U.S. Pat. No. 4,892,105 (toRichard L. Prass), the entire disclosure of which is incorporated hereinby reference. The probe of U.S. Pat. No. 4,892,105 has become known asthe Prass Flush-Tip Monopolar Probe and is insulated up to the distaltip to minimize current shunting through undesired paths. An improvedstructure for a bipolar probe is disclosed in the provisional patentapplication entitled Bipolar Electrical Stimulus Probe (filed Aug. 12,1998, application No. 60/096,243), the entire disclosure of which isalso incorporated herein by reference.

Another method of nerve localization involves mechanical stimulation ofthe nerve of interest by various dissecting instruments. Direct physicalmanipulation of a motor nerve may cause the nerve to conduct a nerveimpulse to its associated musculature. If those muscles are beingmonitored using a nerve integrity monitoring instrument, the surgeonwill hear an acoustic representation of the muscle response in closetemporal relationship to the antecedent mechanical stimulation. Thiswill allow the nerve of interest to be roughly localized at the contactsurface of the dissecting instrument.

Prior art nerve integrity monitoring instruments (such as the Xomed_(C)NIM-2_(C) XL Nerve Integrity Monitor, manufactured by the assignee ofthe present invention) have proven to be effective for performing thebasic functions associated with nerve integrity monitoring such asrecording EMG activity from muscles innervated by an affected nerve andalerting a surgeon when the affected nerve is activated by applicationof a stimulus signal, but have significant limitations for some surgicalapplications and in some operating room environments.

A first problem is users have noticed certain EMG measurement artifactshave a disruptive effect on monitoring and tend to cause undesirablefalse alarms. In particular, EMG monitoring often is performed duringelectrocautery in a surgical procedure, wherein powerful currents surgethrough and cauterize the tissue, often to devastating effect on themonitor's sensitive amplifier circuits. Electrocautery can also inducean undesired direct current (DC) offset from buildup of charge on themonitoring or sensing electrodes or within recording amplifiercircuitry. A method of muting during periods of electrocautery usingin-line detection of electrocautery, based upon frequency and amplitudewas disclosed in Prass, et al.: “Acoustic (Loudspeaker) FacialElectromyographic Monitoring Evoked Electromyographic Activity”,Neurosurgery 19: 392-400, 1986; and an improved method involving aninductive probe pickup was described in U.S. Pat. No. 4,934,377,entitled “Intraoperative Neuroelectro-physiological Monitoring System”,by Prass, et al., the entire disclosures of which are incorporatedherein by reference.

Brief pop noise in the form of high frequency bursts (caused by spuriouselectromagnetic and current artifacts or when non-insulated metalinstruments are accidentally brought into physical contact) may berecorded during nerve integrity monitoring. These brief artifacts may beconfused for true electromyographic (muscle) responses and may lead tomisinterpretation and false alarms, thereby reducing user confidence andsatisfaction in nerve integrity monitoring. Maintenance of highcommon-mode rejection characteristics in the signal conditioning pathhas helped to reduce such interference, however, false alarms stilloccur. Any solution tending to eliminate or minimize false alarmproblems would increase the accuracy and effectiveness of monitoringprocedures.

Prior art nerve integrity monitoring devices incorporate a simplethreshold detection method to identify significant electrical eventsbased upon the amplitude of the signal voltage observed in themonitoring electrodes, relative to a baseline of electrical silence, amethodology having disadvantages for intraoperative nerve integritymonitoring. Use of intramuscular electrodes in close bipolar arrangement(as described in U.S. Pat. No. 5,161,533, cited above) provides adequatespatial selectivity and maintenance of high common mode rejectioncharacteristics in the signal conditioning pathway for reducedinterference by electromagnetic artifacts, but yield a compresseddynamic range of electrical voltage observed between the pairedelectrodes. When physically situated near one of the electrodes, asingle nerve motor unit (e.g., activation of a single nerve fiber) maycause an adequate voltage deflection to be heard (by a surgeon listeningto the EMG audio signal feedback) as a clear signal spike or exceeding apredetermined voltage threshold. Moreover, with close electrode spacingand bipolar amplification, recording of larger responses is frequentlyassociated with internal signal cancellation, significantly reducing theamplitude of the observed electrical signal. The resultant compresseddynamic range is advantageous for supplying direct or raw EMG audiosignal feedback to the operating surgeon, in that both large and smallsignal events may be clearly and comfortably heard at one volumesetting, but an EMG audio signal feedback having compressed dynamicrange provides limited ability to fractionate responses based uponmagnitude of the response or obtain an accurate measure of signal power.Another disadvantage of prior art methodology of threshold detection isthat the surgeon cannot readily distinguish or select between electricalartifacts and EMG activity.

A second problem is that the nerves of interest may frequently exhibit avariable amount of irritability during the surgical procedure, which maybe caused by a disease process or by surgical manipulations such as mildtraction or by drying or thermal effects. Such nerve irritability isrecorded by nerve integrity monitoring electrodes and is displayed andannunciated to the operating surgeon as a series of “beeps” caused byrepetitive triggering of threshold detection or by repetitiveelectromyographic spikes. Because nerve irritability does not appear inclose temporal relationship to particular surgical manipulations, itprovides no localizing information. When such repetitive activity isobserved, the surgeon usually ceases all ongoing surgical manipulationsand may irrigate the surgical field in an attempt to reduce nerveirritability. Once a reasonable effort to reduce nerve irritability hasbeen carried out, any residual nerve irritability becomes “noise” andmay interfere with the ability to detect electrically and mechanicallystimulated nerve activity. Any methods to reduce the effect ofbackground nerve irritability on detection of brief bursts of nerveactivity would enhance localization of nerves of interest during periodsof increased nerve irritability.

A third problem arises when monopolar probes, bipolar probes orelectrified instruments are selected for electrical stimulation duringintraoperative neurophysiological monitoring. Each type of probe has itsown advantages, disadvantages and “best application” duringintraoperative procedures. Because of a variable tendency for currentshunting, the optimum stimulus intensity may vary significantly amongprobes. For a given probe type, the ideal stimulus intensity is lowenough to allow spatial selectivity, but high enough to avoidfalse-negative stimulation as a result of current-shunting or otherinfluences. The commercial EMG-type nerve monitors of the prior art havea single current-source terminating in either one or two outputs. Ifthere are two outputs, the outputs are connected in parallel with asingle common stimulus intensity setting and so there is no ability toprovide separate (optimized) stimulus intensities or to guard againstparallel communication between the two outputs. If both outputs areconnected to stimulus instruments, undetected current-leak could occurthrough parallel channels and result in false-negative stimulation. Atleast one manufacturer of prior art monitoring instruments offers aswitchable connector at the stimulus probe terminus, allowing more thanone stimulus instrument to be kept in readiness, and avoiding parallelconnections to the unused instruments, but performing the act ofswitching requires a surgical staff member such as a nurse or technicianand so is cumbersome and, being time consuming, expensive.

A related problem is that prolonged nerve irritability may be due tolight anesthesia, rather than to inherent nerve irritability. Any methodto distinguish these two possibilities would enhance interpretationduring nerve integrity monitoring.

Another problem confronting users of prior art nerve integritymonitoring devices is that quantitative measurements of nerve functionare relatively cumbersome to obtain, since equipment setting changesmust be performed by operating room personnel while electricalstimulation procedures are performed by the operating surgeon. Forexample, a threshold determination for electrical nerve stimulation isan accepted indication of functional nerve integrity. Determination ofresponse threshold requires stimulation at multiple stimulusintensities, which must be changed manually, and nerve responses must berecorded at each stimulus intensity level. With prior art technology,this process is time-intensive and discourages serial determinationsduring the operation as an ongoing measure of nerve integrity. Thresholddeterminations are typically performed only at the end of the operativeprocedure as a prediction of immediate postoperative function. Whenusing prior art methods, if the threshold is found to be abnormal, thesurgeon is usually unaware of when the change to abnormality occurredduring the operative procedure. Any method making quantitativemeasurements of nerve function convenient and rapid to obtain wouldenhance nerve integrity monitoring.

Another concern is how functions are controlled. There is a relativelystrong conceptual separation between off-line control (performed at sometime other than during the procedure) and on-line control (performedduring a surgical procedure), as pertains to control of intraoperativeneurophysiological monitoring system functions through the use of inputdevices. “Off-line” operations are performed when monitoring is notactively being performed, for example, as when logging-in patientinformation, setting system preferences or retrieving saved-data for“post-production” analysis, whereas “on-line” refers to periods ofactive intraoperative neurophysiological monitoring.

In prior art nerve integrity monitoring devices, controls for off-linefunctions consist of front panel knobs and switches or keyboard andmouse with proprietary software to perform common setup functions andparameter adjustments. Additional back panel switches may be availableto adjust less commonly changed parameters, such as stimulus rate andduration. For multi-channel nerve integrity monitoring with qualitativeand quantitative signal analysis, front and back panel hardware iscumbersome and too limited in scope. Greater flexibility and conveniencein off-line controls is available through use of keyboard and mouseinput and software capabilities to modify and store setup information inarchival files for facilitation of off-line setup functions. Alimitation of prior art strategies is that the setup information is heldin volatile memory during actual monitoring operations, rendering thesetup information vulnerable to strong electrical surges,electromagnetic noise or accidental power interruptions. An electricalsurge or accidental unplugging may cause loss of all new (different from“default”) setup information, requiring a “reboot” of the system andadjustment to get back to the desired settings. Any method for off-linecontrol allowing similar flexibility to a keyboard and mouse input andhaving the convenience of designated software with archival (file)storage of setup information, but without risk of erasure by spuriouselectrical events or accidental equipment unplugging, would represent asignificant advance for nerve integrity monitoring. Prior artstimulation devices for neurophysiological monitoring are manuallycontrolled through front panel potentiometers and switches or with mouseand keyboard to produce paired or burst stimuli and stimuli of oppositepolarity in an alternating pattern, but lack the ability to deliverconsecutive stimuli of differing intensities or alter the pattern ofstimulation at a predetermined time without that time consuming manualinput. Analogously, none of the monitoring instruments of the prior artprovide delivery of selected stimuli in coordination with dataacquisition, analysis, display, and storage. Moreover, in prior artnerve integrity devices, control of on-line functions is performed bykeyboard and mouse or by front panel controls and, because of a possiblebreach of sterility, the operating surgeon cannot perform such functionsby himself or herself and so changing equipment settings requiresinvolvement of hospital personnel at the request of the operatingsurgeon and may be time-consuming, cumbersome and possibly risky, sincethe changed settings may be inaccurate. Any method allowing rapid andaccurate changes in equipment function without the need of ancillaryoperating room personnel and without risk to maintenance of sterilitywould be considered an enhancement of nerve integrity monitoring.

An important function of intraoperative neurophysiological monitoring isdetecting brief episodes of EMG activity, caused by direct electricaland mechanical stimulation. Detection allows the surgeon to localize anerve of interest approximately at the contact surface of the dissectingor stimulating instrument. Detection of brief, localizing EMG activityis frequently obscured by the presence of repetitive EMG activity causedby “baseline” nerve irritability. Such irritability may be due to nervecompromise caused by the disease process itself or to various surgicalmanipulations, such as mild traction, drying, thermal stimulation, orchemical irritation. When significant repetitive activity is observed,the surgeon typically ceases all surgical manipulations and may irrigatethe wound in an attempt to “quiet” nerve irritability. Once a reasonableattempt has been made to allow the nerve to become quieted, anyremaining repetitive activity is essentially “noise” and may interferewith hearing more important brief EMG responses that allow localizationof the nerve of interest. Such background irritability is particularly aproblem during acoustic neuroma resections, which is one of the mostcommon procedures for which facial nerve monitoring is used.

Redundancy afforded by multi-channel monitoring of (single) nerves ofinterest provides some opportunity to maximize the ability to detectlocalizing information during periods of problematic repetitive(non-localizing) activity. The most common application of nerveintegrity monitoring involves monitoring the facial nerve. The facialnerve has a long course, beginning in the cranial cavity, then through abony channel (fallopian canal) within the temporal (ear) bone, exitingbehind the ear to swing forward and innervate the nerves of the facialexpression. The nerve is at risk during a number of surgical proceduresinvolving the ear, the temporal bone and intracranially. Intracranially,and in its course through the temporal bone, the nerve appears as asingle nerve bundle, with no internal topographical organization. As thenerve exits the temporal bone behind the ear it finally separates intotwo major trunks, which further divide into 5 major branches.Multi-channel nerve integrity monitoring of the facial nerve involvesplacing electrodes into multiple facial muscles, representing multiplebranches of the nerve. While not necessarily the preferred approach, thelack of topographical organization of the intracranial and intratemporalportions of the facial nerve, allows monitoring during removal ofacoustic neuromas and during ear surgery with only one or twoelectromyographic channels.

Multichannel monitoring of the facial nerve is preferred in order toincrease sensitivity and to provide redundancy in the event of electrodefailure. Redundant facial nerve monitoring channels also providesflexibility to maximize the ability to detect localizing, briefnon-repetitive EMG activity. The upper and lower facial musculature havebeen observed to have differential tendencies to exhibit mechanicallyevoked EMG activity. The lower face tends to be more sensitive ineliciting mechanically-stimulated EMG activity but also has a greatertendency to exhibit “background” nerve irritability. During periods whenbackground repetitive EMG activity obscures auditory detection of moreimportant and localizing non-repetitive activity, the most active EMGchannels can be deleted (muted) from the signal directed to the surgeonthrough audio loudspeaker(s). The remaining EMG channels, having lesscompeting background noise to interfere, are more easily heard by theoperating surgeon in order to detect (localizing) mechanically andelectrically stimulated EMG activity.

The majority of prior art nerve integrity monitoring devices have onlytwo channels, which allows little redundancy and flexibility. Whenrepetitive activity becomes bothersome and persistent, despitereasonable efforts on the part of the operating surgeon to allow thenerve to quiet down, the surgeon may ask an operating room employee to“turn the monitor down.” This solution is problematic, because it maycause the surgeon to miss hearing important localizing EMG information.Alternatively, with the availability of multiple (redundant) EMGchannels, a nurse or operating room technician may individuallyeliminate each electrode channel in an attempt to identify the offendingchannels, so that they may be (temporarily) eliminated. This process maybe greatly facilitated, if there is some visual indication of relativeEMG activity among the various EMG channels. However, even with visualdisplays, the process may still be time consuming and, therefore,expensive. Moreover, once certain “offending” channels have been muted,there may be long periods before the surgeon, the nurse, or operatingroom technician remember or “feel safe” to add these channels back tothe audio signal. This may cause unnecessarily long periods of decreasedsensitivity.

There is a need, then, for a nerve integrity monitoring instrumenthaving greater flexibility and stability in use, greater sensitivity andspecificity (e.g., noise rejection and artifact identification), and auser interface more readily adapted to performing the monitoringprocedures required without distraction to the surgeon whileconcentrating on the medical aspects of the surgical procedure.

SUMMARY OF THE INVENTION

In accordance with the present invention, an intraoperativeneurophysiological monitoring system includes a number of novelfeatures, including: a digitally controlled stimulator having multipleindependent stimulus outputs; an artifact detection electrode withmodified wire leads to enhance its sensitivity for recording electricalartifacts; a novel method and algorithm for detecting brief artifactsusing the artifact detection electrode and an enhanced method andalgorithm for threshold detection; a method and algorithm forcontrolling the sequence of monitoring events controlled by detection ofprobe contact with tissue; and a method and algorithm for controllingoperation of the nerve integrity monitoring system in which theelectrical stimulus probe is used as a computer pointing or inputdevice.

The intraoperative neurophysiological monitoring system stimulatorpreferably includes a nerve integrity monitoring instrument havingmultiple independent stimulus outputs to provide optimal preset stimulusoutput parameters for more than one probe type, thereby allowing allprobes to be connected at the beginning of the case and used as needed,without delay or confusion related to switching and intensity settingchanges. Independent, electrically isolated outputs also eliminateparallel connections among stimulus probes and possible current leakagebetween probes. An optimum number of stimulus outputs is preferably inthe range of two to four. In an exemplary embodiment, three stimulusoutputs include a monopolar probe, a bipolar probe and an electrifiedinstrument, all three simultaneously connected.

For the purposes of nerve integrity monitoring, an electrical stimulusprobe is used for locating and defining the contour of the nerve ofinterest. During such “mapping” procedures, the stimulus probe is movedabout the surgical field or along the nerve contour in small controlledsteps, during which the stimulus probe is in continuous contact withtissue, usually for less than one or two seconds. Alternatively, duringquantitative measurements of nerve function, the stimulus probe may beapplied to the nerve continuously for a few or several seconds allowingcapture of electromyographic activity for analysis. Thus, if thestimulus probe is in contact with tissue for less than one or twoseconds, it may be taken that the surgeon is simply locating or mappingthe contour of the nerve of interest. If continuous tissue contactexceeds one or two seconds, the surgeon's intent is likely to beotherwise, such as for quantitative measurements. Further, if thestimulus probe is tapped twice or three times onto patient tissue, thetemporal pattern of continuous tissue contact is quite different fromeither of the previous patterns and might be considered as a “request”by the surgeon.

The present invention incorporates a method of controlling a variety ofnerve integrity monitoring functions through detection of the durationof continuous contact of a designated stimulus probe with patienttissue. Alternative methods to more accurately detect the temporalpattern of continuous contact of the stimulus probe with patient tissueinclude continuous measurement of stimulation circuit impedance andmeasurement of current flow using a continuous, distinct (second)subthreshold current, delivered “downstream” from the actual electricalstimulus. Continuous measurement of stimulation circuit impedance is thepreferred method and provides the following benefits:

1. A quality check of the stimulus probe and circuit. Flush tip probes(e.g., as described in U.S. Pat. No. 4,892,105 and provisional patentapplication No. 60/096,243, filed Aug. 12, 1998, the entire disclosuresof which are incorporated herein by reference) have a characteristicimpedance, based partly on the cross-sectional area of the conductor. Ameasured characteristic impedance that is significantly below theexpected characteristic impedance is sensed and indicates a parallelcurrent path or a breach of insulation.

2. Indication of tissue contact with stimulus probe. Detection of tissuecontact is used to drive a preset sequence of events, as discussed ingreater detail, below.

3. A definitive solution for minimizing stimulus-pulse-related recordingartifacts on other equipment by permitting current to flow only duringtissue contact with a stimulus probe. Detection of a characteristicimpedance decrease in the stimulus circuit during tissue contact withthe stimulus probe is sensed and, in response, a relay switch isactivated, allowing current flow to the appropriate probe. If a singlecurrent source is used to drive multiple stimulus outputs, detection ofa characteristic impedance decrease at a stimulus probe output triggersdriving relay switches to “open circuit” other stimulus probe circuitsin order to definitively eliminate parallel connections with otheroutputs.

4. While controversial, constant voltage has been cited as moreadvantageous than constant current for purposes of electricalstimulation, as a means to reduce the occurrence of false-negativestimulation in the setting of stimulus shunting (Moller A, Janotta J.:Preservation of facial function during removal of acoustic neuromas: useof monopolar constant voltage stimulation and EMG. J. Neurosurg 61:757-60, 1984). Since stimulus current is the aspect relating to stimulusadequacy and injury potential, most applications incorporate constantcurrent stimulus sources for greater accuracy and safety in stimulusdelivery. Continuous measurement of stimulus circuit impedance allows a“best of both worlds” opportunity. Stimulus probes and electrifiedinstruments have characteristic or optimal impedance values based uponthe contact surface of the particular instrument. A reduction ofimpedance below that of the characteristic value is taken as anindication of stimulus shunting, presumably away from the area intendedfor electrical stimulation. This is particularly apt to occur with useof electrified instruments, where the insulation is not carried all theway to the tip so as to not interfere with its surgical use. Incombination with digital control of stimulus parameters, detection of astimulus circuit impedance decrease below the pre-determined “optimal”value is used to trigger a compensatory increase in delivered stimuluscurrent in a pre-determined fashion. The rate of change or slope ofcurrent increase, relative to the amount or percentage of impedancedecrease, is preselected for aggressive or less aggressive compensationpatterns and an upper limit of current increase is also predeterminedfor safety considerations. Such a compensatory current increase is saferand more reliable than simple use of constant voltage.

5. The impedance detection circuit provides a mechanism enabling use ofthe stimulator probe as an input device.

Additional circuitry is required for impedance detection, with anadditional patient connection electrode having its own isolation, and anadditional continuous, subthreshold probe signal (i.e., below thethreshold required for nerve activation) must be delivered through theprobe tip for measurement by the impedance detection circuit.

In an alternative embodiment, a continuous, second subthreshold currentis delivered to the stimulus probe, downstream from the pulsed currentused for actual nerve stimulation. Detection of flow of the continuouscurrent provides more accurate detection of tissue contact than forpulsed stimulation alone and permits detecting a “tapping” pattern ofthe stimulus probe. Continuous current flow detection does not provideas many possible benefits as continuous stimulus circuit impedancemeasurement, but also does not require placement of an additionalpatient electrode and the necessary isolation circuitry.

In addition to detecting and responding to a temporal pattern ofcontinuous tissue contact of the stimulus probe, the present stimulatoris adapted for digital control. Stimulus intensity, pulse duration, andtemporal pattern of stimuli presentation are controlled through adigital controller having an interface circuit. The interface storespre-programmed stimulus algorithms or paradigms, preferably innon-volatile memory. The stimulus paradigms are preferably constructedoff-line using appropriate stimulus control algorithm developmentsoftware and is preferably loaded or burned into a non-volatile ReadOnly Memory (ROM) chip, included within the interface. During amonitoring procedure, contact with tissue will trigger a predefinedsequence of events called, for purposes of nomenclature, a TissueContact Initiated (TCI)-Time line, thereby activating the storedstimulus paradigms in a pre-programmed manner.

Front panel controls consist of basic stimulus intensity controls.Stimulus, pulse duration and pulse repetition rate are preferablyadjusted in a limited manner by recessed DIP-switches or otheruser-accessed, but less prominent controls. The remaining stimulatorcontrols are actuated through a CPU interface, such as via a PCI bus. Asdiscussed above, monitoring parameters and complex stimulus paradigmsare stored via non-volatile, programmable memory (e.g., flash memory,EEPROM). The digitally controlled stimulator executing the TCIevent-sequencing time line also communicates with a CPU based datastorage and analysis apparatus to direct binning of responses and totrigger archival data storage, analysis and display paradigms.

In addition to an indication of which stimulator is active and whetheradequate current delivery is achieved, there is preferably also anadditional indicator annunciating detection of an adequate targetimpedance, thereby providing a rough quality check of the stimulus probeand the entire stimulator circuit. This type of diagnostic would be bestapplied to the flush tip stimulus probe designs (as in U.S. Pat. No.4,892,105), where the impedance is typically related to thecross-sectional area of the conductor contact surface.

The controller software used in monitoring the stimulus probe impedancedetection circuit (or current flow detection circuit) includes analgorithm for identifying a pattern of changing impedance (or currentflow change) caused by double or triple taps of the stimulator againstpatient tissue. When double or triple tap patterns are detected, signalsare sent to the circuitry in the CPU digital interface for triggeringpredetermined manipulations. These command signals are preferablyrendered “context sensitive” by their temporal occurrence in relation tothe TCI-Time line.

Turning now to another aspect of the monitoring system of the presentinvention, a method is provided for detection and identification ofartifacts as an aid to interpretation. For the purposes of thisdescription, “intelligent” refers to electrode sites involving important“monitored” muscles, supplied or enervated by a particular nerve ofinterest. Non-intelligent refers to other electrode sites within oroutside of muscles, not supplied by the nerve of interest. Currentartifacts and electromagnetic field noise may best be detected by aspecially constructed electrode that is inserted proximate to therecording field, but not in the (intelligent) muscles supplied by thenerve being monitored. Electrical events, simultaneously recorded inboth “intelligent” electrodes (placed in muscles supplied by the nervebeing monitored) and a “non-intelligent” artifact detection electrode,may be unambiguously interpreted as electrical artifacts. If theartifact detection electrode is placed in a nearby (non-intelligent)muscle not supplied by the nerve being monitored, it may also serve todetect light anesthesia. If repetitive EMG activity is simultaneouslyobserved in monitored muscles and other muscles, it may be interpretedthat the patient is beginning to wake up from anesthesia. Theanesthesiologist may use this information to maintain adequate levels ofanesthesia throughout the procedure. The operating surgeon may also bereassured that the observed nerve irritability is not related tosurgical manipulations. The artifact detection strategy involves theconstruction of an artifact detection electrode, preferably theelectrode of the present invention is a modification of the electrodedesign of U.S. Pat. No. 5,161,533 (as discussed above). The modificationprovides a greater impedance imbalance between the two electrode leads,thereby reliably enhancing the antenna-like qualities of the electrodeand the susceptibility for detecting current and electromagneticartifacts occurring in the immediate proximity of multiple standardelectrodes placed in muscles supplied by the nerve of interest.

The artifact detection electrode of the present invention has an activeportion that is similar to the paired, bipolar Teflon coated needleelectrodes, but differs in that the area of uninsulated needle isdimensioned and/or made of a suitable material to provide a reliablydetectable impedance imbalance.

Preferably, the wire leads are also modified such that the lead lengthis approximately 6 inches longer than standard length. The extra 6-inchportion is looped over the recording field to create, effectively, anantenna over the recording field. The looped portion is treated toenhance its antenna-like properties. Optionally, in combination with orinstead of using differing uninsulated areas of needle insertionportion, a resistor is placed in series with one of the two electrodeleads, thereby creating a readily detected impedance imbalance, thevalue of which may be selected (or, with a potentiometer, adjusted) tobe within a range of, preferably, zero to approximately 50,000 ohms. Theresistor is preferably located on the wire lead or loop, or it may beincorporated into an associated electrical connector housing orconnector body. A relative disadvantage of using a single standardrecording electrode for detection of electromagnetic field and currentartifacts is that the single electrode may not adequately represent theelectromagnetic field for multiple active recording electrodes. The loopdesign, needle to insulation symmetry, fixed resistor value and relativelocation are the physical factors determining the “antenna-like”properties of the electrode design; the various features are preferably“tuned” to obtain the optimum electrode characteristics. The electrodemust be spatially selective enough to avoid pick up of “intelligent”signal, but must have adequate antenna-like qualities to provideEM-field and current artifact detection to represent the entirerecording field.

The uninsulated portion of the electrode needles of the artifactdetection electrode is placed in a proximate, “non-intelligent” muscle,not enervated or supplied by the nerve being monitored. The loopedportion of the electrode lead is placed over the recording field of theintelligent electrodes and held in place, preferably with tape.

The artifact detection electrode output is detected and an algorithmincorporating a simple artifact recognition strategy, based uponresponse distribution, is employed. The signal output of the artifactdetection electrode is amplified along with that of standard“intelligent” electrodes. Brief supra-threshold signal episodes (approx.<1 sec.), detected in intelligent electrodes, trigger a logic-circuit toevaluate for simultaneous signal in the artifact detection electrode.Simultaneous detection of supra-threshold signal in the artifactdetection electrode renders an interpretation of “artifact.” If nosimultaneous signal is detected in the artifact detection electrode, theepisode is interpreted as EMG in the algorithm, since it is highlyunlikely that two different nerves are simultaneously (mechanically orelectrically) stimulated.

For repetitive EMG activity lasting from several seconds to severalminutes, detection of activity among “intelligent” electrodes indicatesirritability in the nerve of interest, which may be due to surgicalmanipulations, whereas simultaneous detection of activity in intelligentand non-intelligent electrodes are interpreted as inadequate or “light”anesthesia, because surgically-evoked repetitive-EMG activity isotherwise unlikely to occur simultaneously in two distinct musclegroups.

An example of such an artifact detection strategy is the use of amasseter muscle electrode during facial nerve monitoring. The massetermuscle is in the proximate electromagnetic field of the facial muscles,but is not enervated by the facial nerve. Brief electromagnetic andcurrent events that are simultaneously detected in facial and massetermuscles are readily interpreted as artifacts. Further, when repetitiveactivity is detected in masseter and facial electrodes, it suggests thatthe anesthesia is getting light.

The intraoperative neurophysiological monitoring system of the presentinvention includes a controller circuit and software algorithms toidentify and categorize artifacts based upon the observed distributionamong “intelligent” and “non-intelligent” electrode sites. In oneembodiment, a logic circuit receives output from threshold detectioncircuits related to both “intelligent” and “non-intelligent” electrodesites. When a supra-threshold signal is detected in one of the“intelligent” electrode sites, the circuit becomes activated to make adetermination regarding whether the signal detected was likely to havebeen artifact or true EMG. At the time of supra-threshold signaldetection in one (or more) of the “intelligent” channels, the output ofthe “non-intelligent” channel threshold detection circuit is checked forsimultaneous activation (using, e.g., a logic AND gate). If there was nosupra-threshold activity in the “non-intelligent” channel, the logiccircuit produces an output signal indicating that the observed activitywas “true EMG”. If simultaneous supra-threshold activity was detected inboth the “intelligent” and “non-intelligent” channels, the logic circuitproduces an output signal indicating that the observed activity waslikely to have been a non-EMG artifact.

The accuracy of the present artifact detection strategy is dependentupon the strength of the recorded signal. Weak signals that only appearin a single channel may not distribute among intelligent andnon-intelligent electrodes as predictably as when multiple electrodesare activated.

If more than one “intelligent” channel (and electrode) is utilized, thelogic circuit is preferably configured to allow a user selectedrequirement to produce an output signal indicating the identity of asupra-threshold signal as “true EMG” or “artifact” only when two or more“intelligent” channels are simultaneously activated by supra-thresholdsignals. This will increase the accuracy of the logic circuitdeterminations, reduce the frequency at which the circuit gives falsepositive feedback, and indicate a response of greater magnitude andprobable significance.

The novel artifact detection electrode and logical strategy fordistinguishing electrical artifacts and EMG signals of the presentinvention works with simple threshold detection involving analog voltagemeasurement, but simple threshold detection has significant limitationsfor this application. One disadvantage is that repetitive EMG activity,caused by persistent nerve irritability, impairs the ability to detectmore important episodes of non-repetitive EMG activity. Repetitiveactivity swamps the threshold detection circuit and causes repetitivedetection of supra-threshold events.

In the present embodiment, threshold detection is improved through theuse of digital signal processing (DSP), whereby all recorded electricalactivity is digitized and evaluated for mathematical properties. Apreferred measurement for EMG activity is rectified root mean square(rRMS), which gives a greater dynamic range for EMG activity magnitude,as detected by standard electrodes (e.g., as in U.S. Pat. No. 5,161,533,discussed above). The greater dynamic range capability improves theability to distinguish responses, based upon the magnitude of signalpower. For example, while electrical artifacts and EMG responses showconsiderable overlap, the peak signal power of a non-repetitive(localizing) EMG activity is usually significantly higher than for arepetitive (non-localizing) EMG activity. The digitally processed rRMSdata stream for each recording channel is continuously analyzed bysoftware for peak and average power within a variable time (probe)window. The width of the probe window (or dwell) over which power isanalyzed may be varied in width (duration) up to one second, which maybe “tuned” to give desired fractionating tendencies. For example, if aminimum average power value is used for determining the event detectionthreshold, a narrow dwell time will reduce the dynamic range and improvedetection of brief responses. Lengthening the dwell time will increasethe dynamic range and favor selection of only larger overall responses.Alternatively, use of peak power determinations effectively neutralizesthe effect of response duration, but may have the greater ability todistinguish repetitive and non-repetitive responses. Predeterminedcriteria for threshold detection may include minimum values for averagepower, peak power or both in some combination or ratio. The use of twodistinct probe windows separated by a variable time (inter-probeinterval) allows greater accuracy in distinguishing brief non-repetitive(<1.0 sec) and longer repetitive (>1.0 sec) electrical events. If theinter-probe interval is selected to be one second, DSP (rRMS) dataappears, via digital scroll, in the second probe window the same as itappeared in the first window, but one second later. A software algorithmmay detect a supra-threshold event in the first probe window andre-analyze it one second later in the second probe window. At the timeof detection of a supra-threshold event in the second window, theactivity in both windows is compared. If there is no supra-thresholdactivity in the first probe window, the activity appearing in the secondwindow had a duration of less than one second. If supra-thresholdactivity occurs simultaneously in both windows, the duration of theactivity observed in the second probe window is taken as equal to orgreater than one second. The inter-probe interval may be varied as ameans to distinguish responses greater than or less than the selectedinterval value. This additional strategy may further enhance the abilityto discretely select which events are to be analyzed by the artifactdetection logical circuit for feedback to the operating surgeon. Asindicated previously, small amplitude responses, which distribute toonly one recording channel, and brief (1.0 sec) repetitive EMG responsesmay be analyzed relatively inaccurately by the present artifactdetection strategy. During surgical procedures, single or weak responsesmay be of important localizing value.

Optionally, additional DSP analysis is used to help distinguishlocalizing non-repetitive EMG activity from electrical artifacts andbrief epochs of repetitive EMG activity. For example, supra-thresholdelectrical events can be captured into a stable buffer for DSP analysis.Additional mathematical treatment of rRMS data is employed foracquisition of additional features which are distinct from thoseselected for general threshold detection purposes. Repetitive EMGactivity typically exhibits a more even power distribution thannon-repetitive EMG activity. A comparison or ratio of peak and averagepower distinguishes the two activities. The values of peak and averagepower required to achieve a reliable fractionation are altered withinthe software and different initial mathematical treatment of DSP data,such as fast Fourier transform, may be useful in separating artifactsand EMG. However, additional DSP methods are presently considered to beless reliable than the use of “intelligent” and “non-intelligent”distributions for distinguishing artifacts and EMG activity. Their useis preferably user enabled and software algorithms are capable ofperiodic updates in order to take advantage of the accumulation ofempirical data.

In one embodiment, the output of an additional DSP analysis is availableas an additional input to the logic circuit, involved with detecting“intelligent” and “non-intelligent” distributions of supra-thresholdevents. Alternatively, outputs of the logic circuit and the additionalDSP may provide input to a separate (third) controller, containingsoftware algorithms for decision making. In either case, the softwarealgorithms may incorporate a hierarchy or system of assigning emphasisor “weight” to various inputs. For example, if electrical activity isdetected simultaneously in the artifact detection electrode with asupra-threshold event detected in a “non-intelligent” location, thisinput suggests that the supra-threshold event was an artifact and mayoverride any other DSP input to the contrary. Alternatively, if therewas no simultaneous activity seen in the non-intelligent electrode, buta supra-threshold event is observed in only one of three or four active“intelligent” channels, the confidence that this is a true EMG responsemay be considerably less assured. In such an instance, a hierarchy maybe constructed within the decision making software algorithm that mayallow certain DSP data to override the initial “verdict,” based uponspatial distribution.

Turning to another aspect of the present invention, as noted above,quantitative measurements of nerve function in intraoperative monitoringare relatively cumbersome and require involvement of technical personnelto change stimulator settings and various recording parameters in orderto acquire, analyze, display and store data. The applicant has notedthat there are not many types of quantitative measurements regardingnerve function assessment, however, and that threshold andpeak-amplitude measurements are the most widely used. The applicant hasalso discovered that paired stimuli pulses are particularly effectivewhen assessing nerve fatigue. Operating surgeons usually have specificpreferences regarding the type of quantitative data to be collected andanalyzed during the course of a given surgical procedure, so there islittle need for “on-the-fly” flexibility in the operating room (OR) whenperforming quantitative data collection.

Quantitative data on nerve function is mainly acquired through the useof an electrical stimulus probe, which provokes electromyographicresponses for quantitative analysis.

The inventor has observed that surgeons use the stimulus probedifferently for locating and “mapping” than for quantitative analysis ofthe functional status of nerves of interest. Temporal aspects ofstimulus probe use can be monitored by the tissue contact detectioncapability within the digital stimulator as described previously. Asignal is generated in the stimulator that relates to the period ofcontinuous contact of the stimulator probe with patient tissue. Thesignal continues as long as continuous tissue contact is maintained andis delivered to a system controller, which is able to initiate multiplepredetermined sequential and parallel operations within the nerveintegrity monitor. These operations relate to delivery of preprogrammedstimulus sequences and to the acquisition, analysis, display andarchival storage of EMG data. Whether the predetermined operations areinitiated or completed depends upon the duration of continuous tissuecontact. For example, if the duration continuous tissue contact is lessthan a preselected period of approximately one or two seconds, thecontroller will maintain the operational status of the nerve integritymonitor in the “search” mode. However, if the duration of continuoustissue contact exceeds the preselected time period, the stimulator orcontroller may alert the surgeon with an indicator tone and thecontroller will automatically change the operational status of the nerveintegrity monitor to a quantitative assessment mode. The indicator tonemay also be designed or configured to signify whether or not adequatecurrent and/or stimulator circuit impedance has been achieved, as anindication of quality assurance.

From the time of tissue contact detection, a digital clock is initiated,controlling a preset sequence of events through a controller interface.For the purposes of this description, the period of continuous tissuecontact of the stimulus probe is termed the “dwell” or “dwell time”, andthe series of preselected operational changes provoked by the “dwell” istermed the “Tissue Contact Initiated Event Sequencing Time line” or“TCI-Time line”. The control method to be described is designed for usewith the main stimulus probe (e.g., stimulus output #1) and may be usedto control all functions of the nerve integrity monitor in a preselectedfashion. The described methodology need not be limited to medicalapplications, in that the use of any probe, where its period of dwellcan be measured, may be similarly configured to control multiplefunctions. The following description involves the preferred embodiment,although many possible sequence strategies are available through theTCI-Time line.

Through the associated controller and controller interface, the onset ofdwell will cause the artifact detection circuit to be suspended(“defeated”) throughout its duration and a preset pattern of stimuluspulses, the intensity of which is determined by front panel controls,will be delivered through the stimulator probe for locating and“mapping” the physical contour of the nerve of interest. After apreselected dwell time of approximately one second, front panel controlof stimulus parameters is defeated, the pattern of stimuli is changedfrom single pulses to alternating paired pulses with single pulses, theintensity of which is somewhat greater (supra-maximal), and the provokedEMG responses are digitized and individually captured into stablebuffers. If the dwell is interrupted before a dwell of 2 seconds, theTCI-Time line is inactivated, the artifact detection circuit is enabled,the stable buffers are cleared of captured signal and pulsed stimuli areno longer delivered through the stimulus probe. After a 2 secondpreselected period of dwell, the controller and associated interfaceinitiate a signal processing sequence, where the captured responses instable buffers are analyzed by averaging the single and paired responsesseparately and computing the difference between the paired and singleresponses by digital subtraction. The magnitude of the single anddigitally subtracted responses are computed and compared. A scalar valuerelating to a ratio of the magnitudes of the digitally subtractedresponse and the single response is stored in a spreadsheet against theabsolute or lapsed time (of the operation) and is displayed by CRToutput automatically or upon an input “request” by the operatingsurgeon. The stable buffers used in these computations are automaticallycleared at completion. The above computational operations occur inparallel to the following.

After a 2 second preselected period of dwell, the controller andinterface defeat front panel control of stimulus parameters and alterthe stimulus delivery pattern to a series of single pulses of varyingintensity. The controller and interface direct the provoked EMGresponses to be captured individually into stable buffers. If the dwellis interrupted prior to completion of the stimulus sequence, theTCI-Time line is discontinued, the sequence of stimulator pulses isdiscontinued, the stable buffers are cleared of captured signal, theartifact detection functions are enabled and stimulus parameters arereverted to front panel controls. However, interruption of the dwellafter 2 seconds does not interfere with the completion of the paralleloperations described above regarding the mathematical treatment of EMGactivity provoked by single and paired stimulus pulses.

If the dwell is continued (after 2 seconds), then until the stimulussequence is completed, the stimulator or TCI-Time line controllerdelivers a second indicator tone and the controller and interfaceinitiate a series of operations to generate a scalar value of responsethreshold. Each individually captured EMG response is analyzed for powercontent (peak or average), the scalar value of which is stored in aspreadsheet in conjunction with the stimulus intensity used to provokeit. The spreadsheet data relating to all stimulus intensities andcorresponding responses is used to compute (or estimate) the stimulusintensity in milliamps (mA) at which half-maximal response magnitude(power) occurred. This scalar value (in mA) is then defined as the“response threshold” and is applied to a spreadsheet against absolute orlapsed time of the surgical procedure. The scalar value or a graphicalplot of threshold versus operative time may be displayed automaticallyby CRT screen or displayed upon request by input supplied by theoperating surgeon. These computational operations are carried out inparallel with progress of the dwell and may reach completionconsiderably after the dwell has been interrupted.

As described, the “TCI-Time line” is a multidimensional controlalgorithm or device utilizing information spanning both time and space.The continuous tissue contact dwell serves to initiate various series ofoperations through the TCI-Time line controller and interface. Theseoperations may include simple or complex stimulus delivery paradigms,and corresponding data acquisition, analysis, display and archivalstorage procedures. The stimulation sequences and data handlingalgorithms proceed along different time lines, as per pre-programmed,parallel (processing) software algorithms. As long as the dwellcontinues, these operations proceed to completion in sequence.Alternatively, interruption of the dwell aborts all subsequentinitiation of events along the dwell, but may allow some of thepreviously initiated events to reach completion as described above. TheTCI-Time line controller directs operational events in differentlocations within the nerve integrity monitoring device. Production ofstimulus pulses occurs in the stimulator portion of the monitor, whiledata acquisition, analysis, display and storage may occur in differentlocations, such as on the memory of a PCI card, CPU RAM memory or a harddrive. Thus the present TCI-Time line control system must account formultiple time dimensions and multiple locations within the monitoringdevice.

Detection of tissue contact is preferably achieved by continuousstimulator circuit impedance measurement or continuous measurement ofcurrent flow with use of a separate sub-threshold current delivereddownstream from actual pulsed stimuli to the patient. Either of thesemethods will allow the detection of the temporal pattern caused bytapping the stimulator probe two or three times onto patient tissue(away from important structures) as a means of providing additionalinput to the controller through the tissue contact detection circuit. A“double” or “triple” tap of the stimulus probe may be preselected foraltering the normal operation of the controller, such as initiating adisplay of previously stored data as a “time trend.” That is, a “doubletap” command may provoke the controller to display a time trend of ameasured parameter, such as response threshold. The scalar value ofstimulus intensity (mA), where the response threshold is achieved, isplotted against time (duration of the operation) to give the surgeon aclearer impression of how the nerve of interest has responded throughoutthe surgical procedure.

Optionally, the control capabilities of the TCI-Time line are used foranalyzing and storing data derived from detection of supra-thresholdevents. Supra-threshold events may be transferred from stable buffers,described previously with regard to “additional DSP” analysis ofsupra-threshold events, and converted to file format for archivalstorage. The file of the digitized signal, its scalar DSP values (e.g.,peak and average rRMS), and its channel number (or identity) may bearchived (as in a spreadsheet) against the absolute or lapsed(operative) time of its appearance for later (off-line) retrieval. Suchcapabilities improve the ability to “tune” DSP parameters for greateraccuracy in detecting appropriate events for analysis, for alerting theoperating surgeon and for distinguishing artifacts from true EMG.

Preferably, audio and video capture devices are integrated into thesystem to perform audio and video data capture functions. An independentmethod of distinguishing artifact and EMG supra-threshold events is tointerpret events in the context of the surgical procedure. If thesupra-threshold event occurred exactly at the time of a surgicalmanipulation, it may be interpreted as a mechanically stimulated (hencenon-repetitive) EMG event. Alternatively, if the event appears to occurindependently of surgical manipulations it is interpreted as eitherartifact or non-localizing (repetitive) EMG. Relatively brief (3-5seconds) periods of digitized audio signal of the sound delivered to thesurgeon through the loudspeaker in the nerve integrity monitor anddigitized video of the surgical procedure, from a (microscope or handheld) camera monitoring the surgical field, is adequate to interpret the“context” of a supra-threshold event. Audio and video signals may bedigitized and held in FIFO “scroll” buffers within the nerve integritymonitor. For investigational purposes, the logical circuits used fordetection of supra-threshold events may send a signal to the TCI-Timeline controller when certain preselected supra-threshold events aredetected; the signal provokes the TCI-Time line controller to cause thecapture of digitized audio and video for an interval starting 2-4seconds before and ending one second after the onset of thesupra-threshold event. The captured audio and video can then beconverted to file form (*.avi, *.mpg or equivalent) and archived alongwith the signal data mentioned above. Such capability tremendouslyfacilitates evaluation (validation) of various methods of event(artifact and EMG response type) detection for accuracy andeffectiveness.

With the present control system, temporal aspects of stimulus probe usecan be made to control an entire quantitative analysis paradigm in apre-programmed, preset manner, based upon the needs of the user. Thiswill involve a mix of sequential and parallel operations and smoothoperation is dependent upon a seamless digital CPU interface for controlof data acquisition, analysis and display, preferably in a Windows®based software system. The algorithm steps or command sequences andinterrupt interpretations are stored in non-volatile memory, such asEEPROM or “flash memory,” providing fast online operation in acontroller which is readily reprogrammed or modified off-line by CPUinterface. At present, the prevailing standard digital interface is thePeripheral Components Interface (PCI); it is to be understood thatfuture developments may provide equivalents to the PCI standard.Accordingly, the following discussion is a description of but oneexemplary embodiment which happens to include a PCI circuit card.

The enhanced or “complete” neurophysiological monitoring system consistsof the basic monitoring unit, a processor including a CPU (e.g., anIntel Pentium® brand microprocessor) and a Peripheral ComponentsInterface (PCI) circuit card. The CPU interfaces with the basicmonitoring unit through the PCI for both off-line and on-lineoperations. Digitized signals from the basic monitoring unit arecontinually delivered (e.g., via an optical transmission link) to thePCI card, which continually routes them to temporary scroll buffers.When triggered by the tissue contact initiated (TCI) Time line or bydetection of evoked EMG responses, recorded signal events are“captured,” along with time, data channel identification and otherrelevant information. The captured signals are held in a stable bufferfor DSP manipulations (e.g., Fast Fourier Transform (FFT) frequencyconversion) and for conversion to a selected file format. A scrollbuffer is a first-in-first-out (FIFO) image buffer storing the mostrecent waveform segment; the stored segment has a selected duration(e.g., approx. 2-10 seconds). A stable buffer (or bin) is also an imagebuffer but only holds discrete supra-threshold waveforms or events, andso effectively ignores the waveform trace between events; the stablebuffer holds waveforms of selected durations or epoch lengths (e.g.,approximately 1 second).

The PCI interface includes the scroll buffers and the stable bufferscontaining captured signal data for quantitative facial nerve signalassessment. Associated DSP circuitry is located on a PCI circuit board.There must be a relatively generous number of stable buffers (or bins)available to separately capture one or more given EMG events on multiplechannels and to capture individual responses relating to stimuli ofdiffering parameters (intensities). Additional buffer spaces or binsmust also be available for digital subtraction functions, where a“third” bin stores the computed difference between two others forfurther quantitative analysis. The total number of bins must be adequateto handle a variety of analysis algorithms.

There must also be consideration for how signals occurringsimultaneously or nearly simultaneously will be processed. Theindividual bin size must be adequate to store a large number of samples,thereby providing adequate waveform fidelity and the sample rate ortime-base must be high enough to capture signals with the requiredaccuracy.

The processor includes a motherboard having a CPU for acquisition ofscalar data from the DSP circuits on the PCI card and for datapresentation functions such as spreadsheeting and graphing (e.g., usingLotus® or Excel® spreadsheet programs) and for controlling the display.The processor is preferably also configured to create, tag and bundledata files from the temporary stable buffers on the PCI card. Image datafiles are preferably bundled with corresponding “captured” audio andvideo files (from separate video and sound cards) and then transferredinto permanent storage in appropriate locations on the processor harddrive for later review. Off-line, saved data is readily re-loaded intotemporary stable buffers to permit the surgeon to review or re-analyzedata to observe the effectiveness of artifact recognition and nervefunction assessment.

Thus, the system delegates DSP functions to various components for rapidperformance of mathematical operations and display of data. Complexstimulation paradigms are initiated by a digitally controlledstimulator, based upon temporal aspects of tissue contact by the mainstimulus probe. The digital stimulator (or the controller executing theTCI-Time line algorithm) sends simultaneous signals through the PCIinterface to direct data to the appropriate buffers (or bins) foron-line analysis. Additional signals, either from the basic monitoringunit or internally generated on the PCI by pre-programmed algorithms,initiate pre-set data display and data storage algorithms. Six to twelvedifferent stimuli and a corresponding number of storage buffers may beemployed for threshold detection. Alternating paired and single pulseswill require at least three bins: one each for binning responses evokedby paired and single pulses, and a third for holding computed digitalsubtraction data. Optionally, within the two bins for single and pairedresponses or by combining the results of separate bins, repetitiousresponses may be used to compute a signal “average” for single andpaired responses. The respective averages may be used to compute thedigital subtraction data for the “third” bin.

Complete control over on-line operations of the intraoperativeneurophysiological monitor of the present invention can be achievedthrough the use of the TCI-Time line and is preferably set up off-lineusing keyboard and mouse input devices through a standard personalcomputer operating system such as Microsoft Windows® software.

A preferred embodiment is that all changes made by off-line inputprocedures are transferred to the main unit of the nerve integritymonitor and “burned in” to non-volatile (EEPROM or flash) memory. As aresult, the information transferred will be protected from spuriousvoltage spikes and accidental unplugging. This is distinct from priorart methodology, where off-line changes are stored in volatile memory,which may be susceptible to spurious voltage spikes and accidentalunplugging of equipment.

Additional on-line flexibility is afforded through use of simple inputdevices which are convenient and easy to use, but not as comprehensiveas the keyboard and mouse combination; in one embodiment, the stimulusprobe is used as a pointing device for inputs to the controller. Duringsurgery, or when “on-line”, an electrical stimulus probe is preferablyemployed as a convenient controller input device and the TCI-Time linealgorithm controls most on-line system operations, including which dataare displayed to the operating surgeon on the CRT screen display,however, the surgeon may periodically want to see additionalinformation, such as a display of a measured parameter graphed as afunction of time, over course of the procedure. The stimulus probeprovides a convenient and simple input device for initiating suchrequests, since the surgeon is likely already holding the probe, and soneed not put the probe down to use a keyboard, or the like.

The TCI-Time line algorithm is triggered upon detection of tissuecontact by the electrical stimulus probe. Tissue contact detectionincludes probe signal current flow or impedance change detection.

In addition to providing an indication of presence or absence of tissuecontact, the tissue contact detection apparatus is configured torecognize specific signatures, such as a “double tap” or “triple tap” ofthe stimulus probe against non-sensitive patient tissue within thesurgical field. The detection of these predetermined signatures can beused to provide additional online input to the TCI-Time line controller.When such a pattern is detected, a separate signal is sent to theTCI-Time line controller for initiation of context sensitive,predetermined commands, a sequence analogous to a “double click” of astandard mouse when pointing to an icon in a Windows® compatibleprogram. The identity of these commands are changeable, depending uponthe monitoring context of the request; context is provided by theTCI-Time line algorithm. If the “double-click” occurs before thecompletion of a TCI-Time line controlled operation, the request isinterpreted differently than for a double-click occurring aftercompletion.

The tapping pattern can differ among different users. In order for thetapping pattern of a given user to be recognized, a setup algorithmincludes an adjustment method allowing the user to input his or herindividual tapping pattern. Recognition of tapping patterns may beperformed by “default” recognition settings within the tissue contactdetection circuitry. However, because the temporal aspects of tappingmay vary significantly among individual surgeons, the preferred systemallows an individual surgeon's tapping signature to be captured forlater recognition. It is preferred that this is performed early in thesurgical procedure, before critical stages. For this procedure, a frontpanel or foot pedal switch is depressed, immediately after which thesurgeon performs a “double tap” or “triple tap” signature. The patternof impedance change or current flow change detected by the tissuecontact detection circuitry is stored and used as a template forrecognition of similar “signature” patterns at a later time.

Also, when the double or triple tap input command is used, a soundsample or audible annunciation is preferably activated to indicate thatthe intended command has been successfully communicated. The soundsample might can be any form of effective audible feedback to the user(e.g., a sound of a standard mouse double-click or triple-click).

After completion of a TCI-Time line controlled, pre-programmed stimulussequence with corresponding quantitative data display, the algorithmpreferably includes program steps for detecting a stimulus probe doubletap and, in response, displaying all similar measurements obtained fromthe beginning of the procedure (e.g., traced as a waveform showingvoltage as a function of time), wherein a time-trend of stimulationthreshold can be observed to detect a significant injury in progress.Similarly, after supra-threshold detection of an EMG response, thealgorithm may include “if then” condition detection program stepswherein detection of a “double tap” is the input causing a display ofthe IDSP data for that response or for a display of a DSP-derivedparameter, such as root mean square (RMS) power, as a function of time.Such a trend may show a loss of signal power over the course of theprocedure and may indicate a fatigue trend in the nerve underobservation in response to ongoing mechanical manipulations.

A simple input device used in conjunction with the TCI-Time linealgorithm alternatively includes two or three button operated switchesaccessed from a cylindrical handle. The two button configuration may beused in a manner similar to setting of a watch; one button selectsoptions from a menu displayed on the nerve integrity monitor and theother button is used to choose a user preference or selection from themenu of options. Alternatively, a three-button input device providesmore flexibility with forward and backward movement through a menu orseries of menus, since the buttons could be used to scroll up, scrolldown or select option, respectively. The simple input device is readilykept sterile in the surgical field and its simplicity allows rapid dataor control input and ease of use. Such a device does not require the useof the stimulating probe.

The above described simple devices for on-line use provide input throughthe monitoring system controller digital interface, rather than througha serial port of the host computer. Off-line operations, controlled bykeyboard and mouse, preferably operate through mouse and keyboard portson the controller CPU.

As discussed above, the intraoperative neurophysiological monitoringsystem also includes an enhanced method and algorithm for detecting orthresholding non-repetitive EMG events or activity (such as the shortduration pulses indicative of EMG activity) as distinguished fromrepetitive EMG activity, even when the non-repetitive EMG events aresensed simultaneously with the repetitive EMG events, in which case thewaveforms are superposed upon one another. The enhanced thresholddetection algorithm includes the steps of buffering or storing acontinuous series of samples of the sensed EMG waveforms from one ormore sensing electrodes; the buffered waveform is processed by runningthe stored waveform samples serially through spaced probefirst-in-first-out (FIFO) sampling windows of selected duration andhaving a selected temporal spacing therebetween; in the preferredembodiment, the probe sampling windows have a duration in the range of0.25 seconds to 0.5 seconds and the beginning of the first probesampling window is temporally spaced at one second from the beginning ofthe second probe sampling window. The algorithm passes the storedwaveform samples serially through the first probe sampling window andthen through the second probe sampling window. As the stored waveformsamples pass through each probe sampling window, a scalar valuecorresponding to the rectified RMS (rRMS) power of the waveform isgenerated. The algorithm continuously computes a threshold value bysubtracting the instantaneous value of the second probe sampling windowrRMS power from the first probe sampling window rRMS power; thecontinuously generated results of this computation are readily plottedas a threshold value waveform. Since the algorithm passes the storedwaveform samples serially through the first probe sampling window andthen through the second probe sampling window, a non-repetitive EMGactivity will produce a threshold value waveform having a first,positive going pulse having a width approximating the duration of thenon-repetitive EMG activity (corresponding to the first probe samplingwindow rRMS power) and then a second negative going pulse having thesame width (corresponding to the subtracted second probe sampling windowrRMS power).

A repetitive EMG activity having a duration longer than the selected(1.0 second) spacing between the probe sampling windows produces athreshold value waveform having only one positive going pulse having awidth approximating the duration of the interval beginning at the startof the first probe sampling window and ending at the start of the secondprobe sampling window (in the present example, a duration of onesecond). For a stored waveform having a non-repetitive EMG activitysuperposed on a repetitive EMG activity, as above, the algorithm willproduce a threshold value waveform having a first one second long pulseincluding a second positive going pulse having a width approximating theduration of the non-repetitive EMG activity (corresponding to the firstprobe sampling window rRMS power) and then a second negative going pulsehaving the same width as the non-repetitive EMG activity (correspondingto the subtracted second probe sampling window rRMS power).

Whenever the enhanced threshold detection algorithm produces a thresholdvalue waveform including a first positive going pulse followed by asecond negative going pulse, there is an indication that a brief (e.g.,<1.0 sec) response has occurred which may be either localizingnon-repetitive EMG or artifact. Detection of such an event provokes theartifact detection circuitry to evaluate its spatial distribution among“intelligent” and “non-intelligent” electrodes and (optionally)additional DSP algorithms in order to determine its status as anartifact or (localizing) EMG event. The surgeon is then prompted with anappropriate audible and (optionally) visual annunciation.

Another aspect of the present invention is a method for reducingirritating and distracting noise from repetitive EMG activity madepossible by the enhanced threshold detection strategy described above.Data from all (and exclusively) “intelligent” EMG channels is digitizedand monitored by the enhanced threshold detection circuit, employing twoprobe windows as described, with an inter-probe interval ofapproximately one second. By DSP, the average rRMS is continuouslycomputed for both windows and the scalar value is referenced againstelectrical silence. With the two probe window strategy, if only onewindow is active at a time, the duration of a supra-threshold event mustbe less than the inter-probe interval. If both windows are activesimultaneously, the duration is equal to or greater than the inter-probeinterval. Since the vast majority of non-repetitive activity is lessthan one second in duration, an inter-probe interval of one second isable to effectively distinguish repetitive and non-repetitive responses.Repetitive responses are detected when both probe windows aresimultaneously active. In the “automatic” squelch embodiment, the scalarvalues of average rRMS derived from the two probe windows arecontinuously scanned by a software comparator constructed innon-volatile memory. The comparator is configured to compare ongoingaverage rRMS values against a user preselected threshold value. If thethreshold value is exceeded in both probe windows, a signal is generatedwhich activates a muting switch to eliminate that particular channelfrom the audio (loudspeaker) signal to the operating surgeon. If otherchannels reach supra-threshold levels of continuous repetitive EMGactivity, more channels may be muted, except the last (quietest)channel. That is, no matter how much repetitive activity, at least one“intelligent” channel is preserved for continuous audio display of EMGsignals to the operating surgeon. When the average rRMS values of bothwindows decrease below threshold levels, the muting switch isautomatically disabled.

In an alternative manual squelch embodiment, the muting function can beenabled manually. Some surgeons may prefer to decide on a “case by case”basis when to begin muting offending EMG channels. When bothered bypersistent repetitive EMG activity, the surgeon may request that a nurseor technician depress a momentary push-button switch, convenientlylocated on the front panel of the nerve integrity monitor. Withactivation of the push-button switch, all (except the quietest) channelswith supra-threshold levels of repetitive EMG activity are muted fromthe audio signal to the surgeon. As with the previous “automatic”embodiment, once the activity has quieted to sub-threshold levels, theaudio output is automatically re-enabled. It is preferred that thesurgeon be given the option of automatic and manual operation by simplefront panel control selections.

Various objects, features and advantages of the present invention willbecome apparent upon consideration of the following detailed descriptionof a specific embodiment thereof, particularly when taken in conjunctionwith the accompanying drawings, wherein like reference numerals in thevarious figures are utilized to designate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the intraoperative neurophysiologicalmonitoring system digitally controlled stimulator impedance and currentflow detection circuit elements, in accordance with the presentinvention.

FIG. 2 is a flow diagram illustrating the impedance detection algorithmfor detection of tissue contact, for use with the monitoring systemstimulator impedance detection circuit of FIG. 1, in accordance with thepresent invention.

FIG. 3 is a flow diagram illustrating the impedance detection algorithmfor detection of tissue contact, for use with the monitoring systemstimulator current and impedance detection circuits of FIG. 1, inaccordance with the present invention.

FIG. 4 is a block diagram of the intraoperative neurophysiologicalmonitoring system digitally controlled stimulator current flow detectioncircuit, in accordance with the present invention.

FIG. 5. is a flow diagram illustrating the current flow detectionalgorithm for detection of tissue contact, for use with the monitoringsystem stimulator current flow detection circuit of FIG. 4, inaccordance with the present invention.

FIG. 6 is a top view of an artifact detection electrode in accordancewith the present invention.

FIG. 7 is a graphical representation of a pre-programmed set ofelectrical stimulus pulses of varying intensities used in theintraoperative monitoring of responses to stimulus pulses, incombination with a flow diagram illustrating the steps in the TCI-Timeline algorithm and the context sensitive interrupt commands forcontrolling whether the TCI Time line algorithm is aborted or completed.

FIG. 8 is a block diagram of the intraoperative neurophysiologicalmonitoring system TCI-Time line algorithm controlled impedance andcurrent flow detection circuit, in accordance with the presentinvention.

FIG. 9 is a block diagram of the intraoperative neurophysiologicalmonitoring system TCI-Time line algorithm controlled current flowdetection trigger circuit, in accordance with the present invention.

FIG. 10 is a set of related waveform traces, plotted as a function oftime, illustrating first and second probe sampling windows each of aselected duration and temporally spaced at a selected inter-probeinterval.

FIG. 11 is a waveform trace, plotted with voltage as a function of time,illustrating a non-repetitive EMG activity.

FIG. 12 is a waveform trace, plotted with voltage as a function of time,illustrating a repetitive EMG activity.

FIG. 13 is a set of related waveform traces, plotted with voltage as afunction of time, illustrating a non-repetitive EMG activity superposedon (or occurring and sensed simultaneously with) a repetitive EMGactivity.

FIG. 14 is a set of related waveform traces, plotted with voltage as afunction of time, illustrating a sensed non-repetitive EMG signaltemporally situated between first and second probe sampling windows ofselected durations, and the rectified RMS power derived from thedifference detection algorithm for first and second probe samplingwindow durations.

FIG. 15 is a set of related waveform traces, plotted with voltage as afunction of time, illustrating a sensed repetitive EMG signal of aduration including first and second probe sampling windows of selecteddurations, and the rectified RMS power derived from the differencedetection algorithm for first and second probe sampling windowdurations.

FIG. 16 is a set of related waveform traces, plotted with voltage as afunction of time, illustrating a sensed non-repetitive EMG signaltemporally situated between first and second probe sampling windows andsuperposed on a sensed repetitive EMG signal of a duration including thefirst and second probe sampling windows, and the rectified RMS powerderived from the difference detection algorithm for a selected probesampling window duration.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring specifically to FIG. 1 of the accompanying drawings, anintraoperative neurophysiological monitoring system 20 includesdigitally controlled stimulator impedance and current flow detectioncircuit elements for use during intraoperative neurophysiologicalmonitoring and preferably in conjunction with a tissue contact initiatedevent sequencing time line algorithm (TCI-Time line) for control of dataacquisition, analysis, display and storage. A current source 20 isconnected with parallel inputs to three electronic switches (“currentgates”) CG-1, CG-2, and CG-3, 24, 26 and 28; although separate currentsources for each stimulus output may be employed, a single currentsource is shown here. Each stimulus output includes a stimuluscontroller (e.g., SC-1) 30 controlling the intensity, duration andtemporal patterns of delivered stimulus pulses. The controller is, inturn, connected with, controlled by and responsive to a digitalinterface (e.g., DI-1) 32 which is in turn connected to a CPU (notshown) and a comparator (e.g., C-C1) 34. Each stimulus output alsoincludes a current detection circuit (e.g., CD-1) 36 and a stimulusisolation unit (e.g., SIU-1) 38. Each stimulus output includes anelectronic switch (e.g., 24) which is responsive to and driven by tissuecontact detection. The present design uses impedance detection as themeans to detect tissue contact. Switches 24, 26 and 28 are kept in theopen circuit position until tissue contact is detected at one of thecathode terminals at the output. Tissue contact produces a signal fromthe corresponding impedance detection circuit to close the electronicswitch, the probe for which is in contact with tissue, and open circuitsthe other switches. Preferably, switches 24, 26 and 28 are configured sothat only one switch (e.g., 24) can be closed at a time. Current flow ismeasured for each stimulus output by a current flow detection circuit(e.g., 36), and the output of circuit 36 drives the digital indicationor read out of current flow for the corresponding stimulus output. Theoutput signal of the current detection circuit 36 is responsive tomeasured current flow and is compared against the “user-intended”current level by comparator circuit 34.

If the current value falls within predetermined limits (90-95%),comparator circuit 34 outputs an “enable” signal, to be used to triggeran “adequate-current” speech sample or tone, thereby providing audiblefeedback for the surgeon. The detection of the enable signal is atriggering event for execution of the TCI-Time line algorithm in the CPUand digital interface 32. Each stimulus output includes a digitalinterface (e.g., 32) storing various stimulus paradigms, which areinitiated in a pre-programmed fashion (as by the TCI-Time linealgorithm) upon tissue contact detection. Digital interface 32 alsodirects data capture, analysis, display and storage in a pre-programmedfashion per the TCI-Time line. The interface 32 consists of twocomponents, one of which is located in a system main monitoring unit,the other of which is located in a PCI bus slot in a system computer.Digital interface 32 employs stimulus controller 30 which shapes thecurrent provided from the current source 22 into stimuli ofpre-programmed intensity, duration, and having a pre-selected temporalpattern. The stimulus controller 30 is driven by digital interface 32,which stores stimulus paradigms in non-volatile memory and initiates thestimulus paradigms as pre-programmed within the TCI-Time line algorithm.Digital interface 32 also controls functions relating to dataacquisition, analysis, display, and storage through its connection withthe CPU. For stimulus output #2, #3 or both, digital interface 32 may beconfigured to input measured values of stimulus circuit impedance andmake pre-programmed adjustments of stimulus intensity, based uponimpedance values. It is anticipated that stimulus output #1 will be usedwith a flush tip stimulus probe for which such an application is notnecessary.

In FIG. 1, impedance detection circuit ID-1, 40 is included to providean indication of tissue contact and to measure nominal stimulus circuitimpedance. Detection of tissue contact is used to initiate the TCI-Timeline algorithm and measurement of impedance is used to provide a“quality-check” of the stimulus circuit integrity and provide a means ofadjusting stimulus intensity to the level of current shunting. Forimpedance measurement, impedance detection circuit 40 provides a small,sub-threshold signal that is detected. The patient connections for theimpedance detection circuit 40 are electrically or optically isolatedfrom the line-powered circuitry by connection through circuit isolationelement I-I1, 42, and are preferably connected to a comparator 44 whichreceives the output of impedance detection circuit 40 and computesscalar representations of measured stimulus circuit impedance.Comparator 44 provides an output for use by the digital interface 32 todrive the various data-handling operations and preprogrammed stimulusintensity adjustments.

Turning now to FIG. 2, a flow diagram illustrates the impedancedetection algorithm for detection of tissue contact, for use with themonitoring system stimulator impedance detection circuit of FIG. 1. Onceprobe 50 is brought into contact with the tissue of a patient, the probeimpedance (which is initially an open circuit impedance at the probetip) is reduced to the measured tissue impedance and switch 24 is closedto provide stimulus current. Substantially simultaneously, the TCI-Timeline algorithm is initiated and a light emitting diode (LED) isilluminated, thus providing an indication of “appropriate” tissuecontact impedance. Stimulus current flows through probe 50 and the nervetissue and the current flow is detected in current detection circuit 36;if the detected current is of the appropriate magnitude, an LED isilluminated, thus providing an indication of “appropriate” current flow.Once the surgeon lifts the probe and interrupts tissue contact, theincreased, open circuit impedance is detected, and switch 24 is opencircuited in response, while the impedance detection circuit 40 isactivated and the TCI-Time line algorithm is reset.

FIG. 3 is a flow diagram illustrating the impedance detection algorithmfor detection of tissue contact, for use with the monitoring systemstimulator current and impedance detection circuits of FIG. 1. Onceprobe 50 is brought into contact with the tissue of a patient, the probeimpedance (which is initially an open circuit impedance at the probetip) is reduced to the measured tissue impedance and switch 24 is closedto provide stimulus current. Substantially simultaneously, the TCI-Timeline algorithm is initiated and a light emitting diode (LED) isilluminated, thus providing an indication of “appropriate” tissuecontact impedance. Stimulus current flows through probe 50 and the nervetissue and the current flow is detected in current detection circuit 36;if the detected current is of the appropriate magnitude, the impedancedetection circuit 40 is open circuited and an LED is illuminated, thusproviding an indication of “appropriate” current flow. Once the surgeonlifts the probe and interrupts tissue contact, current flow stops andthe lack of current flow is detected with current detection circuit 36,whereupon switch 24 is open circuited in response, while the impedancedetection circuit 40 is re-closed and the TCI-Time line algorithm isreset.

The intraoperative neurophysiological monitoring system 20 comprises astimulator that preferably includes a nerve integrity monitoringinstrument having multiple independent stimulus outputs to provideoptimal preset stimulus output parameters for more than one probe type,thereby allowing all probes to be connected at the beginning of the caseand used as needed, without delay or confusion related to switching andintensity setting changes. Independent, electrically isolated outputsalso eliminate parallel connections among stimulus probes and possiblecurrent leakage between probes. In the exemplary embodiment of FIG. 1,three stimulus outputs include a monopolar probe 50, a bipolar probe(not shown) and an electrified instrument (not shown), all threesimultaneously connected.

For the purposes of nerve integrity monitoring, electrical stimulusprobe 50 is used for locating and defining the contour of the nerve ofinterest. During “mapping” procedures, the stimulus probe 50 is movedabout the surgical field or along the nerve contour in small controlledsteps, during which probe 50 is in continuous contact with tissue,usually for less than one or two seconds. Alternatively, duringquantitative measurements of nerve function, probe 50 may be applied tothe nerve continuously for a few or several seconds allowing capture ofelectromyographic activity for analysis. Thus, if probe 50 is in contactwith tissue for less than one or two seconds, it may be taken that thesurgeon is simply locating or mapping the contour of the nerve ofinterest. If continuous tissue contact exceeds one or two seconds, thesurgeon's intent is likely to be otherwise, such as for quantitativemeasurements. Further, if the stimulus probe 50 is tapped twice or threetimes onto patient tissue, the temporal pattern of continuous tissuecontact is different from either of the previous patterns and isconsidered a “request” by the surgeon.

The present invention incorporates a method of controlling a variety ofnerve integrity monitoring functions through detection of the durationof continuous contact of probe 50 with patient tissue. Alternativemethods to more accurately detect the temporal pattern of continuouscontact of probe 50 with patient tissue include continuous measurementof stimulation circuit impedance and measurement of current flow using acontinuous, distinct (second) sub-threshold current, delivered“downstream” from the actual electrical stimulus (e.g., using thedownstream current source, CS-2, as shown in FIG. 4).

Returning to FIG. 1, stimulation circuit impedance is accomplished byusing the impedance detection circuit 40 to enable use of stimulatorprobe 50 as an input device.

Additional circuit elements 40, 42, 44 are required for impedancedetection, with an additional patient connection electrode 54 (e.g., amonopolar subdermal electrode) having its own isolation circuit 42, andan additional continuous, sub-threshold probe signal (i.e., below thethreshold required for nerve activation) must be delivered through theprobe tip for measurement by the impedance detection circuit 42.

In an alternative embodiment of the monitoring system 100 (as shown inFIG. 4), a continuous, second sub-threshold current generated in currentsource #2, CS-2, 104 is delivered to a stimulus probe 102, downstreamfrom the pulsed current used for actual nerve stimulation. Detection offlow of the continuous current provides more accurate detection oftissue contact than for pulsed stimulation alone and permits detecting a“tapping” pattern of the stimulus probe. Continuous current flowdetection does not provide as many possible benefits as continuousstimulus circuit impedance measurement, but also does not requireplacement of an additional patient electrode and the necessary isolationcircuitry.

In addition to detecting and responding to a temporal pattern ofcontinuous tissue contact of the stimulus probe, the stimulators ofFIGS. 1 and 4 are adapted for digital control. Stimulus intensity, pulseduration, and temporal pattern of stimuli presentation are controlledthrough a digital controller having a digital interface circuit 32. Theinterface 32 (and the accompanying CPU) stores pre-programmed stimulusalgorithms or paradigms, preferably in non-volatile memory. The stimulusparadigms are preferably constructed off-line using appropriate stimuluscontrol algorithm development software and are preferably loaded orburned into a non-volatile Read Only Memory (ROM) chip, included withinthe interface. During a monitoring procedure, contact with tissue willtrigger a predefined sequence of events called, for purposes ofnomenclature, a Tissue Contact Initiated (TCI)-Time line, therebyactivating the stored stimulus paradigms in a pre-programmed manner.

Front panel controls for monitoring system 20 consist of basic stimulusintensity controls. Stimulus, pulse duration and pulse repetition rateare preferably adjusted in a limited manner by recessed DIP-switches orother user-accessed, but less prominent controls. The remainingstimulator controls are actuated through the digital CPU interface 32,such as via a PCI bus. As discussed above, monitoring parameters andcomplex stimulus paradigms are stored via non-volatile, programmablememory (e.g., flash memory, EEPROM). The digitally controlled stimulatorexecuting the TCI event-sequencing time line also communicates with aCPU (not shown) based data storage and analysis apparatus to directbinning or storing of responses and to trigger archival data storage,analysis and display paradigms.

In addition to an indication of which stimulator is active and whetheradequate current delivery is achieved, there is preferably also anadditional indicator annunciating detection of an adequate targetimpedance, thereby providing a rough quality check of the stimulus probeand the entire stimulator circuit. This type of diagnostic would be bestapplied to the flush tip stimulus probe designs (as in U.S. Pat. No.4,892,105), where the impedance is typically related to thecross-sectional area of the conductor contact surface.

The controller software used in monitoring the stimulus probe impedancedetection circuit 40 (or current flow detection circuit 36) includes analgorithm for identifying a pattern of changing impedance (or currentflow change) caused by double or triple taps of probe 50 against patienttissue. When double or triple tap patterns are detected, signals aresent to the circuitry in the CPU digital interface 32 for triggeringpredetermined manipulations. These command signals are preferablyrendered “context sensitive” by their temporal occurrence in relation tothe TCI-Time line.

Returning now to FIG. 4, the intraoperative neurophysiologicalmonitoring system 100 includes digitally controlled stimulator currentflow detection circuit CD-1, 106 and a second, downstream current sourceCS-2, 104, and is well suited to performing the method of the presentinvention with current flow detection only. Current source 108 comprisesthe main source of current to provide nerve stimulation; althoughseparate current sources for each stimulus output may be employed, asingle source is shown.

Current Source #2, 104 provides continuous, sub-threshold currentthrough a cathode of stimulus output probe 102 for detection of tissuecontact. As above, the switches or current gates CG-1, CG-2 and CG-3 areactuated or driven by tissue contact detection. The present design usescurrent flow detection as the means to detect tissue contact. Theswitches (“current-gates”) are kept in the open position until tissuecontact is detected at one of the cathode terminals at the output, whichproduces a signal from the corresponding impedance detection circuit toclose the electronic switch for the probe (e.g., 102) in contact withtissue, and opens the others. As above, the switches are configured sothat only one switch can be closed at a time. Each stimulus output alsohas a stimulus controller SC-1 that effects the intensity, duration andtemporal patterns of delivered stimulus pulses. The controller is, inturn, controlled by a digital interface DI-1. Current flow will bemeasured for each stimulus output by a current flow detection circuit(e.g. CD-1). Second current source 104 injects a continuous,sub-threshold current beyond the current-gate CG-1, which is used forthe detection of tissue contact. During delivery of “stimulus current”,the output of the CS-2 circuit is used to drive the digital readout ofcurrent flow for the corresponding stimulus output (e.g., probe 102).

The output of current flow detector (CFD) CD-1, relating to measured“stimulus current” flow, is compared against the “user-intended” levelby a “comparator” circuit 112. If the value falls within predeterminedlimits (90-95%), the comparator circuit 112 puts out an “enable” signalused to trigger an “adequate-current” speech-sample or tone, andpreferably also incorporated as an “enable” signal for the TCI-Timeline.

Digital interface DI-1, 114 stores various stimulus paradigms which areinitiated in a pre-programmed fashion (TCI-Time line) by detection oftissue contact of the primary stimulus probe 102. Digital interface 114also directs data capture, analysis, display and storage in apre-programmed fashion per the TCI-Time line. Interface 114 consists oftwo components, one of which is located in the main unit, the other ofwhich is located in a PCI bus slot in the computer. The digitalinterface 114 controls a stimulus controller SC-1, 116 which shapes thecurrent provided from current source 108 into stimuli of pre-programmedintensity, duration, and temporal pattern. The digital interface DI-1,114 also controls functions relating to data acquisition, analysis,display, and storage through a connection with a CPU (not shown).

For stimulus outputs #2, #3 or both (shown only through the SCsegments), the digital interface may be configured to input measuredvalues of stimulus circuit impedance and make pre-programmed adjustmentsof stimulus intensity, based upon measured impedance values. It isanticipated that stimulus output #1 (shown in its entirety) will be usedwith a flush tip stimulus probe 102 for which such an application isunnecessary.

FIG. 5. is a flow diagram illustrating the current flow detectionalgorithm for detection of tissue contact, for use with the monitoringsystem stimulator current flow detection circuit of FIG. 4, inaccordance with the present invention. Once probe 102 is brought intocontact with the tissue of a patient, a small probe current from secondcurrent source 104 is sensed and current gate or switch CG-1 is closedto provide stimulus current. Substantially simultaneously, the TCI Timeline algorithm is initiated and, optionally, a light emitting diode(LED) is illuminated, thus providing an indication of “appropriate”tissue contact impedance. Stimulus current flows through probe 102 andthe nerve tissue and the stimulus current flow is detected in currentdetection circuit CD-1, 106; if the detected current is of theappropriate magnitude, an LED is illuminated, thus providing anindication of “appropriate” current flow. Once the surgeon lifts theprobe and interrupts tissue contact, current flow stops and the lack ofcurrent flow is detected with current detection circuit 106, whereuponswitch CG-1 is open circuited in response and the TCI-Time linealgorithm is reset.

FIG. 6 is a top view of an artifact detection electrode 130 for useduring intraoperative neurophysiological monitoring to provide areliable means of detecting electromagnetic and current artifacts,occurring in the physical proximity of multiple active recordingelectrodes. Signal output from artifact detection electrode 130 is usedin a simple logic paradigm for the purposes of distinguishingelectromagnetic (EM) and current artifacts from biophysiologicalresponses, and is useful to detect when general anesthesia is becominginadequate or light. Artifact detection electrode 130 is well suited fordetection and identification of artifacts as an aid to interpretation,and can be placed in different groups of muscles to obtain differentmeasurements. For the purposes of this description, “intelligent” refersto electrode sites involving important “monitored” muscles, supplied orenervated by a particular nerve of interest. Non-intelligent refers toother electrode sites, within or outside of muscles, not supplied by thenerve of interest. Current artifacts and electromagnetic field noise maybest be detected by electrode 130 when inserted proximate to therecording field, but not in the (intelligent) muscles supplied by thenerve being monitored. Electrical events, simultaneously recorded inboth “intelligent” electrodes (placed in muscles supplied by the nervebeing monitored) and a “non-intelligent” artifact detection electrode,may be unambiguously interpreted as electrical artifacts. If theartifact detection electrode is placed in a nearby (non-intelligent)muscle not supplied by the nerve being monitored, it may also serve todetect light anesthesia. If repetitive EMG activity is simultaneouslyobserved in monitored muscles and other muscles, it may be interpretedthat the patient is beginning to wake up from anesthesia. Theanesthesiologist may use this information to maintain adequate levels ofanesthesia throughout the procedure. The operating surgeon may also bereassured that the observed nerve irritability is not related tosurgical manipulations. This artifact detection strategy is abetted bythe construction of artifact detection electrode 130 which is amodification of the electrode design of U.S. Pat. No. 5,161,533 (asdiscussed above). The modification provides a greater impedanceimbalance between the two electrode leads 132, 134, thereby reliablyenhancing the antenna-like qualities of the electrode and thesusceptibility for detecting current and electromagnetic artifactsoccurring in the immediate proximity of multiple electrodes placed inmuscles supplied by the nerve of interest.

Artifact detection electrode 130 has an active portion that is similarto the paired, bipolar Teflon coated needle electrodes, but differs inthat the area of uninsulated needle 136, 138 is dimensioned and/or madeof a suitable material to provide a reliably detectable impedanceimbalance.

Preferably, wire leads 132, 134 are also modified such that the leadlength is approximately 6 inches longer than standard length. The extra6-inch portion is looped over the recording field to create,effectively, an antenna 139 over the recording field. The looped portionis treated to enhance its antenna-like properties. Optionally, incombination with or instead of using differing uninsulated areas ofneedle insertion portion, a resistor 140 is placed in series with one ofthe two electrode leads, thereby creating a readily detected impedanceimbalance, the value of which may be selected (or, with a potentiometer,adjusted) to be within a range of, preferably, zero to approximately50,000 ohms. Resistor 140 is preferably located on the wire lead or loop139, or it may be incorporated into an associated electrical connectorhousing or connector body (e.g., 144). A relative disadvantage of usinga single standard recording electrode for detection of electromagneticfield and current artifacts is that the single electrode may notadequately represent the electromagnetic field for multiple activerecording electrodes. The loop design, needle to insulation symmetry,fixed resistor value and relative location are the physical factorsdetermining the “antenna-like” properties of the electrode design; thevarious features are preferably “tuned” to obtain the optimum electrodecharacteristics. The electrode must be spatially selective enough toavoid pick up of “intelligent” signal, but must have adequateantenna-like qualities to provide EM field and current artifactdetection to represent the entire recording field.

The uninsulated portion of the electrode needles 136, 138 of theartifact detection electrode 130 is placed in a proximate,“non-intelligent” muscle, not enervated or supplied by the nerve beingmonitored. The looped portion 139 of the electrode leads is placed overthe recording field of the intelligent electrodes and held in place,preferably with tape.

The artifact detection electrode output is detected and an algorithmincorporating a simple artifact recognition strategy, based uponresponse distribution, is employed. The signal output of the artifactdetection electrode is amplified along with that of standard“intelligent” electrodes. Brief supra-threshold signal episodes (approx.<1 sec.), detected in intelligent electrodes, trigger a logic circuit toevaluate for simultaneous signal in the artifact detection electrode.Simultaneous detection of supra-threshold signal in the artifactdetection electrode renders an interpretation of “artifact.” If nosimultaneous signal is detected in the artifact detection electrode, theepisode is interpreted as EMG in the algorithm, since it is highlyunlikely that two different nerves are simultaneously (mechanically orelectrically) stimulated.

For repetitive EMG activity lasting from several seconds to severalminutes, detection of activity among “intelligent” electrodes indicatesirritability in the nerve of interest, which may be due to surgicalmanipulations, whereas simultaneous detection of activity in intelligentand non-intelligent electrodes are interpreted as inadequate or “light”anesthesia, because surgically-evoked repetitive EMG activity isotherwise unlikely to occur simultaneously in two distinct musclegroups.

An example of such an artifact detection strategy is the use of amasseter muscle electrode during facial nerve monitoring. The massetermuscle is in the proximate electromagnetic field of the facial muscles,but is not enervated by the facial nerve. Brief electromagnetic andcurrent events that are simultaneously detected in facial and massetermuscles are readily interpreted as artifacts. Further, when repetitiveactivity is detected in masseter and facial electrodes, it suggests thatthe anesthesia is getting light.

The intraoperative neurophysiological monitoring system of the presentinvention includes a controller circuit and software algorithms toidentify and categorize artifacts based upon the observed distributionamong “intelligent” and “non-intelligent” electrode sites. In oneembodiment, a logic circuit receives output from threshold detectioncircuits related to both “intelligent” and “non intelligent” electrodesites. When a supra-threshold signal is detected in one of the“intelligent” electrode sites, the circuit becomes activated to make adetermination regarding whether the signal detected was likely to havebeen artifact or true EMG. At the time of supra-threshold signaldetection in one (or more) of the “intelligent” channels, the output ofthe “non intelligent” channel threshold detection circuit is checked forsimultaneous activation (using, e.g., a logic AND gate). If there was nosupra-threshold activity in the “non intelligent” channel, the logiccircuit produces an output signal indicating that the observed activitywas “true EMG”. If simultaneous supra-threshold activity was detected inboth the “intelligent” and “non-intelligent” channels, the logic circuitproduces an output signal indicating that the observed activity waslikely to have been a non-EMG artifact.

The accuracy of the present artifact detection strategy is dependentupon the strength of the recorded signal. Weak signals that only appearin a single channel may not distribute among intelligent andnon-intelligent electrodes as predictably as when multiple electrodesare activated.

If more than one “intelligent” channel (and electrode) is utilized, thelogic circuit is preferably configured to allow a user selectedrequirement to produce an output signal indicating the identity of asupra-threshold signal as “true EMG” or “artifact” only when two or more“intelligent” channels are simultaneously activated by supra-thresholdsignals. This will increase the accuracy of the logic circuitdeterminations, reduce the frequency at which the circuit gives falsepositive feedback, and indicate a response of greater magnitude andprobable significance.

The novel artifact detection electrode and logical strategy fordistinguishing electrical artifacts and EMG signals of the presentinvention works with simple threshold detection involving analog voltagemeasurement, but simple threshold detection has significant limitationsfor this application. One disadvantage is that repetitive EMG activity,caused by persistent nerve irritability, impairs the ability to detectmore important episodes of non-repetitive EMG activity. Repetitiveactivity swamps the threshold detection circuit and causes repetitivedetection of supra-threshold events.

In the present embodiment, threshold detection is improved through theuse of digital signal processing (DSP), whereby all recorded electricalactivity is digitized and evaluated for mathematical properties. Apreferred measurement for EMG activity is rectified root mean square(rRMS), which gives a greater dynamic range for EMG activity magnitude,as detected by standard electrodes (e.g., as in U.S. Pat. No. 5,161,533,discussed above). The greater dynamic range capability improves theability to distinguish responses, based upon the magnitude of signalpower. For example, while electrical artifacts and EMG responses showconsiderable overlap, the peak signal power of a non-repetitive(localizing) EMG activity is usually significantly higher than for arepetitive (non-localizing) EMG activity. The digitally processed rRMSdata stream for each recording channel is continuously analyzed bysoftware for peak and average power within a variable time (probe)window. The width of the probe window (or dwell) over which power isanalyzed may be varied in width (duration) up to one second, which maybe “tuned” to give desired fractionating tendencies.

Another aspect of the present invention is an artifact detection methodfor use during intraoperative neurophysiological monitoring andpreferably in conjunction with electrode 130, which is specificallydesigned and used for detection of artifacts.

Additionally, the present invention involves a circuit that isspecifically designed to identify artifacts based upon the observeddistribution among “intelligent” and “non-intelligent” electrode sites.

A simple logic circuit receives output from threshold detection circuitsrelated to both “intelligent” and “non-intelligent” electrode sites.When a supra-threshold (i.e., over threshold) signal is detected in oneof the “intelligent” electrode sites, the circuit becomes activated tomake a determination regarding whether the signal detected was likely tohave been artifact or true EMG. At the time of supra-threshold signaldetection in one (or more) of the “intelligent” channels, the output ofthe “non-intelligent” channel threshold detection circuit is checked forsimultaneous activation. If there was no supra-threshold activity in the“non-intelligent” channel, the logic circuit will produce an outputsignal, indicating that the observed activity was likely to be true EMG.If simultaneous supra-threshold activity in the “non-intelligent”channel was detected, the circuit will produce an output signal,indicating that the observed activity was likely to have been artifact.

Preferably, more than one “intelligent” channel is utilized and so thelogic circuit is configured to only become activated (i.e., to make alogical determination) when two or more “intelligent” channels aresimultaneously activated, thereby increasing the accuracy of the logiccircuit determinations, reducing the frequency at which the circuitgives feedback, and indicating to the surgeon when there has been a moresignificant response.

If DSP analysis of “intelligent” signals is used for the purpose ofartifact identification, the output of that separate determination maybe fed into the present logic circuit. Depending upon whether or not theDSP-related determination agrees with the present distribution-relateddetermination, the logic circuit output might generate an appropriatesignal to indicate “highly-probable,” “probable,” “possible,” or“inconclusive,” depending upon the differential weighting given to therespective methods of determination.

Turning now to FIGS. 7, 8 and 9, another aspect of the present inventionrelates to a versatile, precise and ergonomic method of control formultiple data-management procedures associated with intraoperativeneurophysiological monitoring. The method (discussed above inconjunction with the TCI-Time line algorithm) involves digital controlof a preprogrammed array of electrical stimuli and a coordinated seriesof data acquisition, analysis, display and storage algorithms initiatedthrough the detection of the temporal pattern of electrical stimulusprobe use and is particularly advantageous in the field ofintraoperative electromyographic (EMG) monitoring in association withperiods of electrical stimulus probe use. Certain aspects of the controlsystem may be linked to supra-threshold detection of EMG or artifactactivity. Moreover, the method and algorithm may be adapted to otherfields in which a probe is used for data acquisition and wheredata-management operations can be linked to monitored aspects of itsuse.

FIG. 7 is a graphical representation of a pre-programmed set ofelectrical stimulus pulses of varying intensities used in theintraoperative monitoring of responses to stimulus pulses, incombination with a flow diagram illustrating the steps in the TCI-Timeline algorithm and the context sensitive interrupt commands forcontrolling whether the TCI-Time line algorithm is aborted or completed.FIG. 8 is a block diagram of an intraoperative neurophysiologicalmonitoring system 200 with a TCI-Time line algorithm controlledimpedance and current flow detection circuit, and FIG. 9 is a blockdiagram of an alternative, simpler embodiment including anintraoperative neurophysiological monitoring system 300 with a TCI-Timeline algorithm controlled current flow detection trigger circuit, inaccordance with the present invention.

Referring now to the upper portion of FIG. 7, showing a graphicalrepresentation of a pre-programmed set of electrical stimulus pulses ofvarying intensities used in the intraoperative monitoring of responsesto stimulus pulses, the vertical axis is graduated in milliamps (mA) ofstimulus current applied through a stimulus probe and the horizontalaxis overhead is a time scale in seconds. A preprogrammed pattern orparadigm of stimulus pulses, as illustrated, preferably includes a firstpair 150 of stimulus pulses spaced at less than 100 mS apart and havingequal amplitudes of approximately 0.10 mA, these are called pairedpulses 150 and are followed at a spacing of approximately 100 mS by asingle pulse 152 having an equal amplitude, 0.10 mA. Preferably, thepattern next includes another set of paired pulses 150, followed inalternate succession by another single pulse 152.

The steps of the algorithm are illustrated in the center portion of FIG.7, in which later steps are below the step before. The TCI-Time linealgorithm has two parallel or simultaneous processes, as will bedescribed in greater detail below.

Returning to FIG. 8, intraoperative neurophysiological monitoring system200 includes current source 208 which generates stimulus current and isconnected to the stimulus controller 210 which controls the intensity,duration and temporal patterns of delivered stimulus pulses. Controller210 is, in turn, responsive to and controlled by a digital interface(DI-1) 204. Current flow is measured for each stimulus output by acurrent flow detection circuit 212, and the output of this circuit willbe used to drive the digital readout of current flow for thecorresponding stimulus output. The output of the current flow detectioncircuit 212, relating to measured current flow, is compared against theuser selected level by a comparator circuit 214, and if the value fallswithin predetermined limits (90-95%), the comparator circuit 214optionally puts out an “enable” signal, to be used to trigger an“adequate-current” speech sample or tone and which may also beincorporated as an “enable” signal for the TCI-Time line. Digitalinterface 204 stores various stimulus paradigms, which are initiated ina pre-programmed fashion (TCI-Time line) by detection of tissue contact.The digital interface also directs data capture, analysis, display andstorage in a preprogrammed fashion per the TCI-Time line. The interfaceconsists of two components, one of which is located in the main unit,the other of which is located in a PCI bus slot in the computer.

For output #2, #3 or both, the digital interface may be configured toinput measured values of stimulus circuit impedance and makepre-programmed adjustments of stimulus intensity, based upon impedancevalues. It is anticipated that stimulus output #1 will be used with aflush tip stimulus probe for which such an application is not necessary.Impedance detection circuit ID-1 220 provides an indication of tissuecontact and measurement of nominal stimulus circuit impedance. Detectionof tissue contact can be used to initiate the TCI-Time line andmeasurement of impedance can be used to provide a “quality-check” of thestimulus circuit integrity and provide a means of adjusting stimulusintensity to the level of current-shunting. For impedance measurement,the impedance detection circuit provides a small, sub-threshold signalthat is detected to establish continuity. Patient connections forimpedance detection circuit 220 are electrically or optically isolatedby isolation circuit 222. A comparator 224 receives output fromimpedance detection circuit 220 and computes scalar representations orvalues of stimulus circuit impedance and provides an output digitalinterface 204 to drive the various data-handling operations andpreprogrammed stimulus intensity adjustments. Digital interface 204connects the stimulator with a CPU 206, so that data acquisition,analysis, display and storage can be coordinated. Digital interface 204and CPU 206 execute spreadsheeting of data and drive a graphic display208 (e.g., a CRT or LCD). Digital interface 204 is preferably configuredto direct the capture of digitally-sampled audio and video datacorresponding to signal data. CPU 206 is preferably programmed to storefiles for later retrieval and “off-line” analysis.

As shown in FIG. 9, intraoperative neurophysiological monitoring system300 includes current source 308 which generates stimulus current and isconnected to the stimulus controller 310 which controls the intensity,duration and temporal patterns of delivered stimulus pulses. Controller310 is, in turn, responsive to and controlled by a digital interface(DI-1) 304. Current source #2, CS-2, 309 injects a small, continuous,sub-threshold current as a probe signal to provide means of tissuecontact detection. Current flow is measured for each stimulus output bya current flow detection circuit 312, the output of this circuit beingused to drive the digital readout of current flow for the correspondingstimulus output. The output of the current flow detection circuit 312,relating to measured current flow, is compared against the user selectedlevel by a comparator circuit 314, and if the value falls withinpredetermined limits (90-95%), the comparator circuit 314 optionallyputs out an “enable” signal, to be used to trigger an “adequate-current”speech sample or tone and which may also be incorporated as an “enable”signal for the TCI-Time line.

As noted above, quantitative measurements of nerve function inintraoperative monitoring are relatively cumbersome and requireinvolvement of technical personnel to change stimulator settings andvarious recording parameters in order to acquire, analyze, display andstore data. The applicant has noted that there are not many types ofquantitative measurements regarding nerve function assessment, however,and that threshold and peak amplitude measurements are the most widelyused. The applicant has also discovered that paired stimuli pulses 150are particularly effective when assessing nerve fatigue. Operatingsurgeons usually have specific preferences regarding the type ofquantitative data to be collected and analyzed during the course of agiven surgical procedure, so there is little need for “on-the-fly”flexibility in the operating room (OR) when performing quantitative datacollection.

Quantitative data on nerve function is mainly acquired through the useof an electrical stimulus probe (e.g., 202 or 302, as shown in FIGS. 8and 9), provoking electromyographic responses for quantitative analysis.

The inventor has observed that surgeons use the stimulus probe (e.g.,202) differently for locating and “mapping” than for quantitativeanalysis of the functional status of nerves of interest. Temporalaspects of stimulus probe use can be monitored by the tissue contactdetection capability within the digital stimulator as describedpreviously. A signal is generated in the stimulator that relates to theperiod of continuous contact of the stimulator probe with patienttissue. The signal continues as long as continuous tissue contact ismaintained and is delivered to a system controller, which is able toinitiate multiple predetermined sequential and parallel operationswithin the nerve integrity monitor, as shown in the central flow chartportion of FIG. 7. These operations relate to delivery of preprogrammedstimulus sequences and to the acquisition, analysis, display andarchival storage of EMG data. Whether the predetermined operations areinitiated or completed depends upon the duration of continuous tissuecontact. For example, as shown in the flowchart portion at the bottom ofFIG. 7, if the duration of continuous tissue contact is less than apreselected period of approximately one or two seconds, the controllerwill maintain the operational status of the nerve integrity monitor inthe “search” mode. However, if the duration of continuous tissue contactexceeds the preselected time period, the stimulator or controller mayalert the surgeon with an indicator tone and the controller willautomatically change the operational status of the nerve integritymonitor to a quantitative assessment mode and provide a preprogrammedsequence of quantitative assessment stimulus pulses 160, as shown in theupper portion of FIG. 7. A monitoring indicator tone may also bedesigned or configured to signify whether adequate current and/orstimulator circuit impedance has been achieved, as an indication ofquality assurance, as discussed above. From the time of tissue contactdetection, a digital clock is initiated, controlling a preprogrammedsequence of events through a controller interface DI-1 (as shown inFIGS. 8 and 9). For the purposes of this description, the period ofcontinuous tissue contact of the stimulus probe is termed the “dwell” or“dwell time”, and the series of preselected operational changes provokedby the “dwell” is termed, the “Tissue Contact Initiated Event SequencingTime line” or “TCI-Time line” and is illustrated, in part, in the middleand lower flow charts included in FIG. 7. The control method to bedescribed is designed for use with the main stimulus probe (e.g., probes202, 302 connected to stimulus output #1 in a multi-stimulus outputsystem, as described above) and may be used to control all functions ofthe nerve integrity monitor in a preselected fashion, by execution of analgorithm stored in memory. The described methodology need not belimited to medical applications, in that the use of any probe, where itsperiod of dwell can be measured, may be similarly configured to controlmultiple functions. The following description involves the preferredembodiment, although many possible sequence strategies are availablethrough the TCI-Time line.

Through the associated controller and controller interface (e.g. thedigital interface DI-1 204, of FIGS. 8 and 304, of FIG. 9), the onset ofdwell causes the artifact detection circuit to be suspended (“defeated”)throughout its duration and a preset pattern of stimulus pulses, theintensity of which is determined by front panel controls, will bedelivered through the stimulator probe for locating and “mapping’ thephysical contour of the nerve of interest. After a preselected dwelltime of approximately one second (as shown in the upper part of FIG. 7),front panel control of stimulus parameters is defeated, the pattern ofstimuli is changed from single pulses 152 to alternating paired pulses150 with single pulses 152, the intensity of which is somewhat greater(supra maximal), and the provoked EMG responses are digitized andindividually captured into stable buffers. If the dwell is interruptedbefore a dwell of 2 seconds, the TCI-Time line is inactivated oraborted, the artifact detection circuit is enabled, the stable buffersare cleared of captured responses, and pulsed stimuli are no longerdelivered through the stimulus probe. After a 2 second preselectedperiod of dwell, the controller and associated interface initiate asignal processing sequence, where the captured responses in stablebuffers are analyzed by averaging the single and paired responsesseparately and computing the difference between the paired and singleresponse by digital subtraction. The magnitude of the single anddigitally subtracted responses are computed and compared. A scalar valuerelating to a ratio of the magnitudes of the digitally subtractedresponse and the single response is stored in a spreadsheet against theabsolute or lapsed time (of the operation) and is displayed by CRToutput automatically or upon an input “request” by the operatingsurgeon. The stable buffers used in these computations are automaticallycleared at completion. The above computational operations occur inparallel to the following.

After a 2 second preselected period of dwell, the controller andinterface defeat front panel control of stimulus parameters and alterthe stimulus delivery pattern to a series of single pulses of varyingintensity 160. The controller and interface direct the provoked EMGresponses to be captured individually into stable buffers. If the dwellis interrupted prior to completion of the stimulus sequence, theTCI-Time line is discontinued, the sequence of stimulator pulses isdiscontinued, the stable buffers are cleared of captured responses, theartifact detection functions are enabled and stimulus parameters arereverted to front panel controls. However, interruption of the dwellafter 2 seconds does not interfere with the completion of the paralleloperations described above regarding the mathematical treatment of EMGactivity provoked by single and paired stimulus pulses.

If the dwell is continued (after 2 seconds), then until the stimulussequence is completed, the stimulator or TCI-Time line controllerdelivers a second indicator tone and the controller and interfaceinitiate a series of operations to generate a scalar value of responsethreshold. Each individually captured EMG response is analyzed for powercontent (peak or average), the scalar value of which is stored in aspreadsheet in conjunction with the stimulus intensity used to provokeit. The spreadsheet data relating to all stimulus intensities andcorresponding responses is used to compute (or estimate) the stimulusintensity in milliamps (mA) at which half-maximal response magnitude(power) occurred. This scalar value (in mA) is then defined as the“response threshold” and is applied to a spreadsheet against absolute orlapsed time of the surgical procedure. The scalar value or a graphicalplot of threshold versus operative time may be displayed automaticallyby CRT screen or displayed upon request by input supplied by theoperating surgeon. These computational operations are carried out inparallel with progress of the dwell and may reach completionconsiderably after the dwell has been interrupted.

As described, the “TCI-Time line” is a multidimensional controlalgorithm or device utilizing information spanning both time and space.The continuous tissue contact dwell serves to initiate various series ofoperations through the TCI-Time line controller and interface. Theseoperations may include simple or complex stimulus delivery paradigms,and corresponding data acquisition, analysis, display and archivalstorage procedures. The stimulation sequences and data handlingalgorithms proceed along different time lines, as per pre-programmed,parallel (processing) software algorithms. As long as the dwellcontinues, these operations proceed to completion in sequence.Alternatively, interruption of the dwell aborts all subsequentinitiation of events along the dwell, but may allow some of thepreviously initiated events to reach completion as described above. TheTCI-Time line controller directs operational events in differentlocations within the nerve integrity monitoring device. Production ofstimulus pulses occurs in the stimulator portion of the monitor, whiledata acquisition, analysis, display and storage may occur in differentlocations, such as on the memory of a PCI card, CPU RAM memory or a harddrive. Thus the present TCI-Time line control system must account formultiple time dimensions and multiple locations within the monitoringdevice.

Detection of tissue contact is preferably achieved by continuousstimulator circuit impedance measurement or continuous measurement ofcurrent flow with use of a separate sub-threshold current delivereddownstream from actual pulsed stimuli to the patient. Either of thesemethods will allow the detection of the temporal pattern caused bytapping the stimulator probe two or three times onto patient tissue(away from important structures) as a means of providing additionalinput to the controller through the tissue contact detection circuit. A“double” or “triple” tap of the stimulus probe may be preselected foraltering the normal operation of the controller, such as initiating adisplay of previously stored data as a “time trend”. That is, a “doubletap” command may provoke the controller to display a time trend of ameasured parameter, such as response threshold. The scalar value ofstimulus intensity (mA), where the response threshold is achieved, isplotted against time (duration of the operation) to give the surgeon aclearer impression of how the nerve of interest has responded throughoutthe surgical procedure.

Optionally, the control capabilities of the TCI-Time line are used foranalyzing and storing data derived from detection of supra-thresholdevents. Supra-threshold events may be transferred from stable buffers,described previously with regard to “additional DSP” analysis ofsupra-threshold events, and converted to file format for archivalstorage. The file of the digitized signal, its scalar DSP values (e.g.,peak and average rRMS), and its channel number (or identity) may bearchived (as in a spreadsheet) against the absolute or lapsed(operative) time of its appearance for later (off-line) retrieval. Suchcapabilities improve the ability to “tune” DSP parameters for greateraccuracy in detecting appropriate events for analysis, for alerting theoperating surgeon and for distinguishing artifacts from true EMG.

Preferably, audio and video capture devices are integrated into thesystem to perform audio and video data capture functions. An independentmethod of distinguishing artifact and EMG supra-threshold events is tointerpret events in the context of the surgical procedure. If thesupra-threshold event occurred exactly at the time of a surgicalmanipulation, it may be interpreted as a mechanically stimulated (hencenon-repetitive) EMG event. Alternatively, if the event appears to occurindependently of surgical manipulations it is interpreted as eitherartifact or non-localizing (repetitive) EMG. Relatively brief (3-5seconds) periods of digitized audio signal of the sound delivered to thesurgeon through the loudspeaker in the nerve integrity monitor anddigitized video of the surgical procedure, from a (microscope or handheld) camera monitoring the surgical field, is adequate to interpret the“context” of a supra-threshold event. Audio and video signals may bedigitized and held in FIFO “scroll” buffers within the nerve integritymonitor. For investigational purposes, the logic circuits used fordetection of supra-threshold events may send a signal to the TCI-Timeline controller when certain preselected supra-threshold events aredetected; the signal provokes the TCI-Time line controller to cause thecapture of digitized audio and video for an interval starting 2-4seconds before and ending one second after the onset of thesupra-threshold event. The captured audio and video can then beconverted to file form (*.avi, *.mpg or equivalent) and archived alongwith the signal data mentioned above. Such capability tremendouslyfacilitates evaluation (validation) of various methods of event(artifact and EMG response type) detection for accuracy andeffectiveness.

With the present control system, temporal aspects of stimulus probe usecan be made to control an entire quantitative analysis paradigm in apre-programmed, preset manner, based upon the needs of the user. Thiswill involve a mix of sequential and parallel operations and smoothoperation is dependent upon a seamless digital CPU interface (e.g., 204or 304) for control of data acquisition, analysis and display,preferably in a Windows® based software system. The algorithm steps orcommand sequences and interrupt interpretations are stored innon-volatile memory, such as EEPROM or “flash memory,” providing fastonline operation in a controller which is readily reprogrammed ormodified off-line by CPU-interface. At present, the prevailing standarddigital interface is the Peripheral Components Interface (PCI); it is tobe understood that future developments may provide equivalents to thePCI standard. Accordingly, the following discussion is a description ofbut one exemplary embodiment which happens to include a PCI circuitcard.

The enhanced or “complete” neurophysiological monitoring system 200 (asshown in FIG. 8) consists of the basic monitoring unit, a processor 204including a CPU 206 (e.g., an Intel Pentium® brand microprocessor) and aPeripheral Components Interface (PCI) circuit card. CPU 206 interfaceswith the basic monitoring unit through the PCI for both off-line andon-line operations. Digitized signals from the basic monitoring unit arecontinually delivered (e.g., via an optical transmission link) to thePCI card, which continually routes them to temporary scroll buffers.When triggered by the tissue contact initiated (TCI) Time line or bydetection of evoked EMG responses, recorded signal events are“captured”, along with time, data channel identification and otherrelevant information. The captured signals are held in a stable bufferfor DSP manipulations (e.g., Fast Fourier Transform (FFT) frequencyconversion) and for conversion to a selected file format. A scrollbuffer is a first-in-first-out (FIFO) image buffer storing the mostrecent waveform segment; the stored segment has a selected duration(e.g., approx. 2-10 seconds). A stable buffer (or bin) is also an imagebuffer but only holds discrete supra-threshold waveforms or events, andso effectively ignores the waveform trace between events; the stablebuffer holds waveforms of selected durations or epoch lengths (e.g.,approximately 1 second).

The PCI interface includes the scroll buffers and the stable bufferscontaining captured signal data for quantitative facial nerve signalassessment. Associated DSP circuitry is located on a PCI circuit board.There must be a relatively generous number of stable buffers (or bins)available to separately capture one or more given EMG events on multiplechannels and to capture individual responses relating to stimuli ofdiffering parameters (intensities). Additional buffer spaces or binsmust also be available for digital subtraction functions, where a“third” bin stores the computed difference between two others forfurther quantitative analysis. The total number of bins must be adequateto handle a variety of analysis algorithms.

There must also be consideration for how signals occurringsimultaneously or nearly simultaneously will be processed. Theindividual bin size must be adequate to store a large number of samples,thereby providing adequate waveform fidelity and the sample rate ortime-base must be high enough to capture signals with the requiredaccuracy.

The processor includes a motherboard having a CPU for acquisition ofscalar data from the DSP circuits on the PCI card and for datapresentation functions such as spreadsheeting and graphing (e.g., usingLotus® or Excel® spreadsheet programs) and for controlling the display.The processor is preferably also configured to create, tag and bundledata files from the temporary stable buffers on the PCI card. Image datafiles are preferably bundled with corresponding “captured” audio andvideo files (from separate video and sound cards) and then transferredinto permanent storage in appropriate locations on the processor harddrive for later review. Off-line, saved data is readily re-loaded intotemporary stable buffers to permit the surgeon to review or re-analyzedata to observe the effectiveness of artifact recognition and nervefunction assessment.

Thus, the system delegates DSP functions to various components for rapidperformance of mathematical operations and display of data. Complexstimulation paradigms in the form of software algorithms are initiatedby a digitally controlled stimulator, based upon temporal aspects oftissue contact by the main stimulus probe. The digital stimulator (orthe controller executing the TCI-Time line algorithm) sends simultaneoussignals through the PCI interface to direct data to the appropriatebuffers (or bins) for on-line analysis. Additional signals, either fromthe basic monitoring unit or internally generated on the PCI bypre-programmed algorithms, initiate pre-set data display and datastorage algorithms. Six to twelve different stimuli and a correspondingnumber of storage buffers may be employed for threshold detection.Alternating paired and single pulses will require at least three bins:one each for binning responses evoked by paired and single pulses, and athird for holding computed digital subtraction data. Optionally, withinthe two bins for single and paired responses or by combining the resultsof separate bins, repetitious responses may be used to compute a signal“average” for single and paired responses. The respective averages maybe used to compute the digital subtraction data for the “third” bin.

Complete control over on-line operations of the intraoperativeneurophysiological monitor of the present invention can be achievedthrough the use of the TCI-Time line and is preferably set up off-lineusing keyboard and mouse input devices through a standard personalcomputer operating system such as Microsoft Windows® software.

In the preferred embodiment all changes made by off-line inputprocedures are transferred to the main unit of the nerve integritymonitor and “burned in” to non-volatile (EEPROM or flash) memory. As aresult, the information transferred will be protected from spuriousvoltage spikes and accidental unplugging. This is distinct from priorart methodology, where off-line changes are stored in volatile memory,which may be susceptible to spurious voltage spikes and accidentalunplugging of equipment.

Additional on-line flexibility is afforded through use of simple inputdevices which are convenient and easy to use, but not as comprehensiveas the keyboard and mouse combination; in one embodiment, the stimulusprobe is used as a pointing device for inputs to the controller, as willbe described in greater detail below.

Yet another aspect of the present invention is an adaptable thresholdlevel setting method for use during intraoperative neurophysiologicalmonitoring and is also intended to enhance detection of brief episodesof EMG activity, provoked by mechanical or electrical stimuli. Themethod improves the performance and accuracy of the above describedartifact detection strategy, based upon response distribution among“intelligent” and “non-intelligent” electrodes.

Threshold detection is based on measured signal power (such asroot-mean-square) is monitored for each channel. As signal powerincreases, the threshold is automatically elevated in order to avoidthreshold detection of background EMG activity. One embodiment of thismethod is to sample signal power at intervals, and to hold thedeterminations in temporary memory, such as in a digital scroll method.In order for a supra-threshold signal event to be detected, one or moreconsecutive signal power determinations would have to be greater, by apreset difference level, than the signal power sampled one second beforethem. An alternative is to require that one or more consecutive signalpower determinations be greater than the power levels one second beforeand one second after, thereby limiting threshold detection to just briefresponses.

In an alternative and preferred embodiment, the thresholding could beperformed in conjunction with determination of the likelihood that anobserved event is non-repetitive EMG activity instead of the morenoise-like repetitive EMG activity. The method includes defining“probes” or sampling windows of time which stored waveform traces arepassed through. FIG. 10 is a set of related waveform traces, plotted asa function of time, illustrating first and second probe sampling windows350, 352 each of a selected duration and temporally spaced at a selectedinter-probe interval 354. FIG. 11 is a waveform trace, plotted withvoltage as a function of time, illustrating a non-repetitive EMGactivity 360, and FIG. 12 is a waveform trace, plotted with voltage as afunction of time, illustrating a repetitive EMG activity 362. FIG. 13 isa set of related waveform traces, plotted with voltage as a function oftime, illustrating a non-repetitive EMG activity 360 superposed on (oroccurring and sensed simultaneously with) a repetitive EMG activity 362.FIG. 14 is a set of related waveform traces, plotted with voltage as afunction of time, illustrating a sensed non-repetitive EMG signaltemporally situated between first and second probe sampling windows ofselected durations, and the rectified RMS power derived from thedifference detection algorithm for first and second probe samplingwindow durations. FIG. 15 is a set of related waveform traces, plottedwith voltage as a function of time, illustrating a sensed repetitive EMGsignal of a duration including first and second probe sampling windowsof selected durations, and the rectified RMS power derived from thedifference detection algorithm, for first and second probe samplingwindow durations. FIG. 16 is a set of related waveform traces, plottedwith voltage as a function of time, illustrating a sensed non-repetitiveEMG signal temporally situated between first and second probe samplingwindows and superposed on a sensed repetitive EMG signal of a durationincluding the first and second probe sampling windows, and the rectifiedRMS power derived from the difference detection algorithm, for aselected probe sampling window duration. As discussed above, theintraoperative neurophysiological monitoring system also includes anenhanced method and algorithm for detecting or thresholdingnon-repetitive EMG events or activity (such as the short duration pulses360 indicative of EMG activity) as distinguished from a repetitive EMGactivity waveform 362, even when the non-repetitive EMG events aresensed simultaneously with the repetitive EMG events, in which case thewaveforms are superposed upon one another as shown in FIG. 13. Theenhanced threshold detection algorithm includes the steps of bufferingor storing a continuous series of samples of the sensed EMG waveformsfrom one or more sensing electrodes (e.g., 130). The buffered waveformis processed by running the stored waveform samples serially throughspaced probe first-in-first-out (FIFO) sampling windows of selectedduration and having a selected temporal spacing therebetween. In thepreferred embodiment, the probe sampling windows (e.g., 350, 352) have aduration in the range of 0.25 seconds to 0.5 seconds and the beginningof the first probe sampling window is temporally spaced (with aninter-probe interval 354) at one second from the beginning of the secondprobe sampling window. The algorithm passes the stored waveform samplesserially through first probe sampling window and then through the secondprobe sampling window. As the stored waveform samples (e.g., 360) passthrough each probe sampling window (350, 352), a scalar valuecorresponding to the rectified RMS (rRMS) power of the waveform isgenerated. As best seen in FIGS. 14, 15 and 16, the algorithmcontinuously computes a threshold value by subtracting the instantaneousvalue of the second probe sampling window rRMS power from the firstprobe sampling window rRMS power. The continuously generated results ofthis computation are readily plotted as a threshold value waveform 370.Since the algorithm passes the stored waveform samples serially throughthe first probe sampling window and then through the second probesampling window, a non-repetitive EMG activity will produce thethreshold value waveform 370 (of FIG. 14) having a first, positive goingpulse 372 having a width approximating the duration of thenon-repetitive EMG activity 360 (corresponding to the first probesampling window rRMS power) and then a second negative going pulse 374having the same width (corresponding to the subtracted second probesampling window rRMS power).

Alternatively, as best seen in FIG. 15, a repetitive EMG activity 362having a duration longer than the selected (1.0 second) spacing betweenthe probe sampling windows produces a threshold value waveform 380having only one positive going pulse having a width approximating theduration of the interval beginning at the start of the first probesampling window 350 and ending at the start of the second probe samplingwindow 352 (in the present example, a duration of one second). For astored waveform having a non-repetitive EMG activity superposed on arepetitive EMG activity, as shown in FIG. 16, the algorithm will producea threshold value waveform 384 having a first one second long pulseincluding a second positive going pulse having a width approximating theduration of the non-repetitive EMG activity (corresponding to the firstprobe sampling window rRMS power) and then a second negative going pulsehaving the same width as the non-repetitive EMG activity (correspondingto the subtracted second probe sampling window rRMS power).

Whenever the enhanced threshold detection algorithm produces a thresholdvalue waveform including a first positive going pulse followed by asecond negative going pulse, there is an indication that a brief (e.g.,<1.0 sec) response has occurred which may be either localizingnon-repetitive EMG or artifact. Detection of such an event provokes theartifact detection circuitry to evaluate its spatial distribution among“intelligent” and “non-intelligent” electrodes and (optionally)additional DSP algorithms in order to determine its status as anartifact or (localizing) EMG event. The surgeon is then prompted with anappropriate audible and (optionally) visual annunciation.

Insofar as monitoring instrument use is concerned, additional on-lineflexibility is afforded through use of simple input devices which areconvenient and easy to use, but not as comprehensive as the keyboard andmouse combination. In one embodiment, the stimulus probe (e.g., 202) isused as a pointing device for inputs to the controller. During surgery,or when “on-line”, an electrical stimulus probe is preferably employedas a convenient controller input device and the TCI-Time line algorithmcontrols most on-line system operations, including which data aredisplayed to the operating surgeon on the CRT screen display. However,the surgeon may periodically want to see additional information, such asa display of a measured parameter graphed as a function of time, overcourse of the procedure. The stimulus probe provides a convenient andsimple input device for initiating such requests, since the surgeon islikely already holding the probe, and so need not put the probe down touse a keyboard, or the like. The TCI-Time line algorithm is triggeredupon detection of tissue contact by the electrical stimulus probe.Tissue contact detection includes probe signal current flow or impedancechange detection.

In addition to providing an indication of presence or absence of tissuecontact, the tissue contact detection apparatus is configured torecognize specific signatures, such as a “double tap” or “triple tap” ofthe stimulus probe against non-sensitive patient tissue within thesurgical field. The detection of these predetermined signatures can beused to provide additional online input to the TCI-Time line controller.When such a pattern is detected, a separate signal is sent to theTCI-Time line controller for initiation of context sensitive,predetermined commands, a sequence analogous to a “double click” of astandard mouse when pointing to an icon in a Windows® compatibleprogram. The identity of these commands are changeable, depending uponthe monitoring context of the request; context is provided by theTCI-Time line algorithm. If the “double-click” occurs before thecompletion of a TCI-Time line controlled operation, the request isinterpreted differently than for a double-click occurring aftercompletion.

The tapping pattern can differ among different users. In order for thetapping pattern of a given user to be recognized, a setup algorithmincludes an adjustment method allowing the user to input his or herindividual tapping pattern. Recognition of tapping patterns may beperformed by “default” recognition settings within the tissue contactdetection circuitry. However, because the temporal aspects of tappingmay vary significantly among individual surgeons, the preferred systemallows an individual surgeon's tapping signature to be captured forlater recognition. It is preferred that this is performed early in thesurgical procedure, before critical stages. For this procedure, a frontpanel or foot pedal switch is depressed, immediately after which thesurgeon performs a “double tap” or “triple tap” signature. The patternof impedance change or current flow change detected by the tissuecontact detection circuitry is stored and used as a template forrecognition of similar “signature” patterns at a later time.

Also, when the double or triple tap input command is used, a soundsample or audible annunciation is preferably activated to indicate thatthe intended command has been successfully communicated. The soundsample might can be any form of effective audible feedback to the user(e.g., a sound of a standard mouse double-click or triple-click).

A double tap is defined as a first probe contact having a first, shortduration (e.g., less than one second) followed by an interval in whichthe probe is lifted and not in contact with anything and followed by asecond probe contact having a second, short duration (e.g., also lessthan a second). A triple tap is one in which the second probe contact isfollowed by another interval in which the probe is lifted and not incontact with anything and followed by a third probe contact having athird, short duration (e.g., less than one second). A computerexecutable algorithm (preferably part of the TCI-Time line algorithm)for detecting and responding to the tapping sequence is readily preparedand responds to sensed current or impedance changes indicating that one,two or three taps has occurred, and then, in response, triggersexecution of a desired monitoring or data handling or display orientedcommand.

After completion of a TCI-Time line controlled, pre-programmed stimulussequence with corresponding quantitative data display, the algorithmpreferably includes program steps for detecting a stimulus probe doubletap and, in response, displaying all similar measurements obtained fromthe beginning of the procedure (e.g., traced as a waveform showingvoltage as a function of time), wherein a time-trend of stimulationthreshold can be observed to detect a significant injury in progress.Similarly, after supra-threshold detection of an EMG response, thealgorithm may include “if then” condition detection program stepswherein detection of a “double tap” is the input causing a display ofthe IDSP data for that response or for a display of a DSP-derivedparameter, such as root mean square (RMS) power, as a function of time.Such a trend may show a loss of signal power over the course of theprocedure and may indicate a fatigue trend in the nerve underobservation in response to ongoing mechanical manipulations.

A simple input device used in conjunction with the TCI-Time linealgorithm alternatively includes two or three button operated switchesaccessed from a cylindrical handle. The two button configuration is usedin a manner similar to setting of a watch; one button selects optionsfrom a menu displayed on the nerve integrity monitor and the otherbutton is used to choose a user preference or selection from the menu ofoptions. Alternatively, a three button input device provides moreflexibility with forward and backward movement through a menu or seriesof menus, since the buttons could be used to scroll up, scroll down orselect option, respectively. The simple input device is readily keptsterile in the field and its simplicity allows rapid data or controlinput and ease of use. Such a device does not require the use of thestimulating probe.

The above described simple devices for on-line use provide input throughthe monitoring system controller digital interface, rather than througha serial port of the host computer. Off-line operations, controlled bykeyboard and mouse, preferably operate through mouse and keyboard portson the controller CPU.

Turning now to another aspect of the present invention, a squelchcontrol method is provided for use during multi-channel intraoperativeneurophysiological monitoring for the purposes of enhancing thesurgeon's ability to hear brief localizing (non-repetitive)electromyographic responses during periods of significant backgroundactivity. The squelch control method is based upon the method fordetecting repetitive EMG activity made possible by the enhancedthreshold detection strategy described above. Data from all (andexclusively) “intelligent” EMG channels is digitized and monitored bythe enhanced threshold detection circuit, employing two probe windows asdescribed, with an inter-probe interval of approximately one second. ByDSP, the average rRMS is continuously computed for both windows and thescalar value is referenced against electrical silence. With the twoprobe window strategy, if only one window is active at a time, theduration of a supra-threshold event must be less than the inter-probeinterval. If both windows are active simultaneously, the duration isequal to or greater than the inter-probe interval. Since the vastmajority of non-repetitive activity is less than one second in duration,an inter-probe interval of one second is able to effectively distinguishrepetitive and non-repetitive responses. Repetitive responses aredetected when both probe windows are simultaneously active.

In the “automatic” embodiment of the present invention, the scalarvalues of average rRMS derived from the two probe windows arecontinuously scanned by a software comparator constructed innon-volatile memory. The comparator is configured to compare ongoingaverage rRMS values against a user preselected threshold value. If thethreshold value is exceeded in both probe windows, a signal is generatedwhich activates a muting switch to eliminate that particular channelfrom the audio (loudspeaker) signal to the operating surgeon. If otherchannels reach supra-threshold levels of continuous repetitive EMGactivity, more channels may be muted, except the last (quietest)channel. That is, no matter how much repetitive activity, at least one“intelligent” channel is preserved for continuous audio display of EMGsignals to the operating surgeon. When the average rRMS values of bothwindows decrease below threshold levels, the muting switch isautomatically disabled.

In an alternative embodiment, the muting function can be enabledmanually. Some surgeons may prefer to decide on a “case by case” basis,when to begin muting offending EMG channels. When bothered by persistentrepetitive EMG activity, the surgeon may request that a nurse ortechnician depress a momentary push-button switch, conveniently locatedon the front panel of the nerve integrity monitor. With activation ofthe push button switch, all (except the quietest) channels withsupra-threshold levels of repetitive EMG activity are muted from theaudio signal to the surgeon. As with the previous “automatic”embodiment, once the activity has quieted to sub-threshold levels, theaudio output is automatically re-enabled. It is preferred that thesurgeon be given the option of automatic and manual operation by asimple front panel control selections.

Having described preferred embodiments of a new and improved method, itis believed that other modifications, variations and changes will besuggested to those skilled in the art in view of the teachings set forthherein. It is therefore to be understood that all such variations,modifications and changes are believed to fall within the scope of thepresent invention as defined by the appended claims.

1. An artifact detection electrode for sensing neurophysiological signalartifacts in tissue structures, in the body, comprising: a firstelectrode needle having an insulated proximal portion and a sharp,uninsulated distal portion; a second electrode needle having aninsulated proximal portion and a sharp, uninsulated distal portion; anelectrically conductive, first elongate wire lead having a proximal end,an intermediate portion and a distal end; an electrically conductive,second elongate wire lead having a proximal end, an intermediate portionand a distal end; said first elongate wire lead being electricallyconnected to said first electrode needle proximal portion at said firstelongate wire lead distal end; said second elongate wire lead beingelectrically connected to said second electrode needle proximal portionat said second elongate wire lead distal end; said first elongate wirelead having a circuit element connected in series therewith along saidintermediate portion of said first elongate wire lead; and said firstelongate wire lead intermediate portion and said second elongate wirelead intermediate portion being configured in a loop to define arecording field signal receiving antenna.
 2. The artifact detectionelectrode of claim 1, said circuit element being a circuit element forcreating a detectable impedance imbalance.
 3. The artifact detectionelectrode of claim 1, said first elongate wire lead and said secondelongate wire lead being twisted together distally and proximally ofsaid loop, said loop being defined by segments of said first and secondelongate wire leads which are not twisted together.
 4. An artifactdetection electrode for sensing neurophysiological signal artifacts intissue structures, in the body, comprising: a first electrode needlehaving an insulated proximal portion and a sharp, uninsulated distalportion; a second electrode needle having an insulated proximal portionand a sharp, uninsulated distal portion; a first elongate conductorhaving a proximal end, an intermediate portion and a distal end; asecond elongate conductor having a proximal end, an intermediate portionand a distal end; said first elongate conductor being electricallyconnected to said first electrode needle proximal portion at said firstelongate conductor distal end; said second elongate conductor beingelectrically connected to said second electrode needle proximal portionat said second elongate conductor distal end; said first elongateconductor having a circuit element connected in series therewith, saidfirst elongate conductor having said circuit element connected in seriesin said first elongate conductor intermediate portion; and said firstelongate conductor intermediate portion and said second elongateconductor intermediate portion being configured in a loop to define arecording field signal receiving antenna.
 5. An artifact detectionelectrode for sensing neurophysiological signal artifacts—in tissuestructures, in the body, comprising: a first electrode needle having aninsulated proximal portion and a sharp, uninsulated distal portion; asecond electrode needle having an insulated proximal portion and asharp, uninsulated distal portion; a first elongate conductor having aproximal end, an intermediate portion and a distal end; a secondelongate conductor having a proximal end, an intermediate portion and adistal end; said first elongate conductor being electrically connectedto said first electrode needle proximal portion at said first elongateconductor distal end; said second elongate conductor being electricallyconnected to said second electrode needle proximal portion at saidsecond elongate conductor distal end; said first elongate conductorhaving a circuit element connected in series therewith, said firstelongate conductor having said circuit element disposed in a connectorbody affixed to said proximal end of said first elongate conductor; andsaid first elongate conductor intermediate portion and said secondelongate conductor intermediate portion being configured in a loop todefine a recording field signal receiving antenna.
 6. An artifactdetection electrode for sensing neurophysiological signal artifacts intissue structures, in the body, comprising: a first electrode needlehaving an insulated proximal portion and a sharp, uninsulated distalportion, a second electrode needle having an insulated proximal portionand a sharp, uninsulated distal portion; a first elongate conductorhaving a proximal end, an intermediate portion and a distal end; asecond elongate conductor having a proximal end, an intermediate portionand a distal end; said first elongate conductor being electricallyconnected to said first electrode needle proximal portion at said firstelongate conductor distal end; said second elongate conductor beingelectrically connected to said second electrode needle proximal portionat said second elongate conductor distal end; said first elongateconductor having a resistive circuit element connected in seriestherewith, said resistive circuit element being a fixed value resistor;and said first elongate conductor intermediate portion and said secondelongate conductor intermediate portion being configured in a loop todefine a recording field signal receiving antenna.
 7. An artifactdetection electrode for sensing neurophysiological signal artifacts intissue structures, in the body, comprising: a first electrode needlehaving an insulated proximal portion and a sharp, uninsulated distalportion; a second electrode needle having an insulated proximal portionand a sharp, uninsulated distal portion; a first elongate conductorhaving a proximal end, an intermediate portion and a distal end; asecond elongate conductor having a proximal end, an intermediate portionand a distal end; said first elongate conductor being electricallyconnected to said first electrode needle proximal portion at said firstelongate conductor distal end; said second elongate conductor beingelectrically connected to said second electrode needle proximal portionat said second elongate conductor distal end; said first elongateconductor having a resistive circuit element connected in seriestherewith, said resistive circuit element being a variable resistancepotentiometer; and said first elongate conductor intermediate portionand said second elongate conductor intermediate portion being configuredin a loop to define a recording field signal receiving antenna.
 8. Theartifact detection electrode for sensing neurophysiological signalartifacts of claim 7, said variable resistance potentiometer having anadjustment range of from zero ohms to approximately fifty thousand ohms.